Harden sample concurrency, cancellation, and failure handling

[cdcavell]

Summary

• replace blocking concurrency gates with asynchronous TaskCompletionSource coordination in capability and acknowledgment samples
• propagate test cancellation tokens through concurrent gateway attempts
• honor cancellation before handling or writing centralized error responses
• prevent middleware samples from exposing exception messages to callers
• classify cross-system executor failures into stable categories
• record internal failure diagnostics through ActivitySource and tracing without leaking exception details across the gateway boundary
• document Release configuration as the preferred repository test mode

Validation

• Release build passed
• all 300 sample tests passed
• 0 failed
• 0 skipped