[vMCP] Handle auth credential expiration with retry and proactive refresh

Depends on #3865

With session-scoped clients, short-lived outgoing credentials (e.g., OAuth tokens) resolved at client creation can become stale mid-session. Implement automatic client recreation on auth failure with proper safeguards.

Phase 1 (this issue):

• Detect 401/403 from backend; recreate client with fresh credentials from OutgoingAuthRegistry
• Retry with exponential backoff, max 3 attempts
• Circuit-breaker: after N consecutive auth failures per backend, disable retries and log a warning
• Use singleflight to deduplicate concurrent recreation attempts for the same backend
• Auth-failure retries add non-trivial latency — surface this overhead in distributed traces

Phase 2 (follow-up, separate issue):

• Modify identityPropagatingRoundTripper to read identity from request context per-operation, eliminating the stale credential problem entirely
• For credentials with a known expiry (JWT exp, OAuth expires_in): schedule proactive client recreation before expiry

Acceptance Criteria

• A 401 or 403 from a backend triggers client recreation with fresh credentials
• Recreation is retried with exponential backoff up to a maximum of 3 attempts
• After N consecutive auth failures for a backend, the circuit breaker opens and further retries are skipped
• Concurrent auth failures for the same backend result in only one recreation attempt (via singleflight)
• Auth-failure retry latency is visible in distributed traces
• The raw credentials are never logged
• Unit tests cover: successful recreation, max retries exhausted, circuit breaker opens, concurrent failures deduplicated

RFC: THV-0038 — Session-scoped client lifecycle

Start with RFC THV-0038 and the session-scoped client lifecycle described here, then trace how OutgoingAuthRegistry supplies credentials and how identityPropagatingRoundTripper is used. The Phase 1 work is complete when auth failures recreate clients safely, retry with backoff and circuit-breaking, deduplicate concurrent recreation, expose retry latency in traces, avoid logging raw credentials, and pass the listed unit-test scenarios.