feat(loadtesting): add Load Testing capability index (regression)

[sourabhd-cbu]

What

Adds capability/loadtesting.capability-index.json — the Load Testing product, authored in the per-product capability-index format introduced by #387.

• 19 capabilities over the load-api /api/v1/agent/* surface, 9 entities.
• base_url baked to regression (https://load-testing-rengg-lts.bsstag.com). Companion branch feat/loadtesting-capability-index-prod carries the production host.

Format alignment with #387

• Per-product file under capability/ (not the root combined capability-index.json).
• Envelope { schema_version: 1, version: "1.0", build_id, loadtesting: {...} } — product at top level, no products wrapper.
• Product key order summary, base_url, capabilities, schemas, responses, paging, entities.
• Capabilities carry returns and a responses map referencing named responses. LTS uses one uniform response envelope ({success, data} / {success, errorCode, error}), so schemas holds SuccessEnvelope + ErrorResponse and responses maps the standard codes — plus 503 for insights and 409 for the versioned update.
• Modes: 15 read, 4 write, 0 destructive (no delete routes).

Note on base_url (deliberate divergence from tm)

tm omits base_url because it is region-sharded (EU/IN/US) and a single origin would misroute non-US accounts. LTS is not region-sharded — it has fixed per-environment hosts — so this branch bakes the regression host and its sibling bakes production. If you'd prefer config-based resolution (CAPABILITY_REGISTRY_BASE_URL_LOADTESTING[_ENV]) like tm, I'll drop base_url and collapse to a single branch.

Caveats

1. Hand-authored, not pipeline-generated. Add the tm capability index #387 notes the index is a build artifact from agent-platform-harness-common. This is a hand-authored equivalent following the same shape; the durable path is an LTS product source the export pipeline consumes — happy to move it there.
2. Invoke needs a load-api auth change. The registry forwards Api-Token: <username>:<accessKey>, but load-api's /api/v1/agent/* agentAuth accepts HTTP Basic only. Discovery + host resolution work with this PR; invokeEndpoint will 401 until agentAuth accepts Api-Token (tracked on the load-testing-backend side).

Ref: LTAI-270. This is the regression variant; do not merge into the same deployment as the prod variant — they differ only in base_url.

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