feat(agent): harden PSU gRPC POC support

[Marc-André Moreau (mamoreau-devolutions)]

Hardens the PSU gRPC agent POC and adds optional AppToken support for authenticating the agent to PowerShell Universal. The POC container also avoids running as root and no longer disables TLS verification during Docker builds.

This keeps unauthenticated local development possible when no AppToken is configured, while allowing PSU deployments to pass an application token through agent configuration or the container helper.

Issue: N/A

[Marc-André Moreau (mamoreau-devolutions)]

Implementation notes:

• Adds optional PsuGrpcAgent.AppToken config and sends it as authorization: Bearer <token> gRPC metadata only when configured.
• Adds PSU_APP_TOKEN and -AppToken wiring for the POC container scripts.
• Reworks process stop handling so graceful stop closes stdin while preserving later kill control, and cleans registry state after process failures.
• Reports spawn failures with StreamClosed and ProcessCompleted.
• Drains stdout/stderr pump tasks before completion to avoid losing final output.
• Removes GIT_SSL_NO_VERIFY=true from the POC Docker build and runs the runtime container as a non-root user.
• Validated with cargo check -p devolutions-agent, cargo test -p devolutions-agent --quiet, cargo clippy -p devolutions-agent --tests -- -D warnings, Rust fmt, and script parser checks.

Remaining follow-up work:

• Enforce TLS/HTTPS for AppToken-bearing production connections.
• Add command/runtime allowlisting and process-tree cancellation.
• Tighten protocol semantics around stream kinds, structured status, heartbeat, and cancel response.