feat(python-sdk): support OIDC Bearer auth on SandboxClient

PR NVIDIA#1596 hardened the gateway side of the OIDC story; the Python SDK
was the remaining gap — it only supported plaintext or mTLS, with no
Bearer metadata anywhere. Deployments with OIDC enabled (the
recommended posture since PR NVIDIA#935 / PR NVIDIA#1404) were unreachable from
the SDK.

Adds:

- `bearer_token: str | Callable[[], str] | None` kwarg on
  `SandboxClient`. Static strings or zero-arg callables (the latter
  is invoked per RPC, so callers can drop in a refresh loop or
  token-file watcher without reconstructing the client). Composes
  with `tls` for OIDC-over-mTLS deployments.
- `_BearerAuthInterceptor` implementing all four
  `grpc.{Unary,Stream}{Unary,Stream}ClientInterceptor` types.
  Appends `authorization: Bearer <token>` to outgoing metadata.
  Implemented as an interceptor (not call credentials) so it works
  on both plaintext (`disableTls=true` dev) and TLS channels without
  `grpc.composite_channel_credentials`.
- `TlsConfig` ergonomics: all three fields (`ca_path`, `cert_path`,
  `key_path`) are now optional with `cert_path` / `key_path`
  required-together-or-not-at-all (enforced in `__post_init__`). This
  unlocks three transport profiles from one dataclass:
    * full mTLS (all three)
    * CA-only trust (`ca_path` only)
    * system roots (`TlsConfig()` — for OIDC gateways behind a
      public CA)
- `from_active_cluster` mirrors `crates/openshell-tui/src/lib.rs`
  `build_oidc_channel`:
    * For any `https://` gateway, always build a secure channel.
      Pick the strongest TLS profile available in `mtls/` (full
      mTLS → CA-only → system roots). No more `insecure_channel`
      fallback for HTTPS.
    * Gate OIDC bearer attachment on
      `metadata.json["auth_mode"] == "oidc"`. Matches
      `crates/openshell-cli/src/main.rs:132` and the TUI; a stale
      `oidc_token.json` next to a non-OIDC gateway no longer causes
      the SDK to attach a bearer.
- `_OidcRefresher` — thread-safe, in-process native OAuth2 refresh
  modeled on `google.oauth2.credentials.Credentials` and
  `botocore.tokens.SSOTokenProvider`. Lazily checks expiry on every
  RPC; when stale, re-reads disk first (the CLI may have rotated
  the bundle), and only then exchanges the refresh_token against
  the IdP's token endpoint discovered via OIDC discovery
  (`/.well-known/openid-configuration`, cached after first call).
  Concurrent RPCs share a single refresh via `threading.Lock` (no
  IdP stampede). Honors refresh-token rotation. Surfaces IdP
  failures as `SandboxError` with the RFC 6749 error body included
  for diagnostics.

  Mirrors the Rust CLI's HTTP-policy posture from
  `crates/openshell-cli/src/oidc_auth.rs`:
    * `follow_redirects=False` so a 3xx during discovery can't
      steer us to an attacker-controlled token endpoint.
    * Discovery `issuer` is validated against the configured
      issuer; a discovery document claiming a different issuer is
      rejected, preventing the SDK from POSTing the refresh_token
      to a malicious endpoint.
    * `insecure: bool` flag plumbed through to httpx's
      `verify=` so self-signed-cert deployments work the same way
      they do in the Rust CLI.

  Built on `httpx` (chosen over `urllib` specifically for
  follow_redirects + verify control as kwargs). The OAuth2
  refresh-token grant itself (RFC 6749 §6) is one form-encoded
  POST — handled inline rather than via a dedicated OAuth library;
  tried `authlib`'s `OAuth2Client` first but it auto-injects an
  Authorization header on every request, which breaks the
  unauthenticated discovery GET.
- `_make_cluster_bearer_provider(..., auto_refresh=True,
  write_back=True, insecure=False)` factory. Defaults to the
  refresher path with write-back enabled; `auto_refresh=False`
  falls back to the read-only fail-closed behavior for callers that
  don't want the SDK to make outbound HTTP calls to the IdP.

  `write_back=True` is the default (changed from the first round of
  review): IdPs with refresh-token rotation (Keycloak with
  rotation, Entra in strict mode) invalidate the old refresh_token
  on each refresh, so an in-memory-only refresh would leave the
  on-disk bundle pointing at an invalidated value — any second
  process starting from disk would `invalid_grant`. With write-back
  enabled by default, the SDK keeps the shared cache consistent
  with the IdP.
- `from_active_cluster` exposes `auto_refresh`, `write_back`, and
  `insecure` kwargs (defaults: True / True / False).

OAuth2 refresh policy and write-back semantics deliberately mirror
what the major Python SDKs do — see
github.com/googleapis/google-auth-library-python (`Credentials`)
and github.com/boto/botocore (`SSOTokenProvider`):

| Library                       | Native refresh | Writes back |
|-------------------------------|----------------|-------------|
| google-auth Credentials       | yes            | no          |
| botocore SSOTokenProvider     | yes            | yes         |
| openshell SandboxClient (here)| yes (opt-out)  | yes (opt-out)|

OpenShell sits between the two; chose write-back-by-default because
the rotation invariant matters more for our deployments than the
"CLI is the only writer" assumption that fits google-auth.

Adds `httpx>=0.27` as a runtime dependency. No new OAuth2 library —
the refresh grant is a single POST.

Tested:

- 36 SDK unit tests pass (5 existing + 31 across the bearer
  interceptor, fail-closed provider, refresher behavior, TlsConfig
  validation, from_active_cluster auth ladder, and three
  regression tests for the security review findings).
  `mise run test:python` → 44 passed total.
- `mise run python:lint` (ruff) clean.
- End-to-end against a Keycloak-protected gateway on OpenShift:
    * unauthenticated `Health` bypass works
    * admin + `openshell:all` reaches user-callable methods
    * reader (`sandbox:read`) denied on `CreateSandbox` by scope
    * admin + `openshell:all` denied on PR NVIDIA#1596 sandbox-only
      methods at the router (the new gate is honored from the SDK)
    * full provider CRUD lifecycle via the SDK
    * callable token provider rotates per RPC as expected
- Regression-probed against three pre-review security findings:
    * **Discovery issuer validation** — a discovery document
      claiming a different `issuer` than the configured one is
      rejected with a clear `SandboxError` before any refresh POST
      can reach the attacker-controlled endpoint.
    * **Redirect during discovery** — `follow_redirects=False` on
      the underlying httpx client means a 3xx during discovery
      surfaces as a SandboxError rather than silently chasing the
      redirect.
    * **Cross-process rotation** — a two-process simulation shows
      process B starting from disk and successfully refreshing
      with the rotated refresh_token, because process A's
      write-back updated the shared cache.
- Refresher unit tests cover: cached-fresh fast path, disk-rotated
  re-read before refresh, OAuth2 exchange against the discovered
  token endpoint, refresh-token rotation, atomic write-back at
  0600 mode (default), default-on write_back proven by test,
  concurrent N-thread coordination (one refresh shared across 8
  threads), IdP failure surfaced with the error body, the
  client_credentials / no-refresh_token error path, issuer-
  mismatch rejection, redirect-during-discovery rejection,
  insecure flag plumbing.

Signed-off-by: Mrunal Patel <mrunalp@gmail.com>

Author: mrunalp

pyproject.toml

@@ -18,6 +18,25 @@ dependencies = [
     "grpcio>=1.60",
     "protobuf>=4.25",
 ]
+
+[project.optional-dependencies]
+# OIDC bearer-auth refresh path. Required only when:
+#   - using SandboxClient.from_active_cluster() against a gateway with
+#     metadata.auth_mode == "oidc" and auto_refresh=True (the default), OR
+#   - explicitly constructing _OidcRefresher.
+# Plaintext / mTLS-only deployments don't need this extra.
+#
+# Install with: pip install "openshell[oidc]"
+oidc = [
+    # HTTP client used by the refresh path. Chosen over urllib for
+    # explicit redirect control (`follow_redirects=False`) and clean
+    # SSL-verification toggle (`verify=`), both of which are
+    # security-critical for OIDC discovery and the refresh-token POST.
+    # The OAuth2 refresh-token grant itself (RFC 6749 §6) is a single
+    # form-encoded POST — small enough to spell out directly without a
+    # dedicated OAuth library.
+    "httpx>=0.27",
+]
 classifiers = [
     "Development Status :: 3 - Alpha",
     "Intended Audience :: Developers",