fix(cag): resolve data races, add queue observability, fix test coverage

Cycle 2 fixes from reviewer-sre + reviewer-quality:

- scheduler.go: remove delete() under RLock (data race); expired entries
  skipped on read, cleaned lazily by next MarkFailed write
- proxy.go: pre-populate transports for scheduler backends at startup to
  prevent concurrent map writes from ServeHTTP goroutines
- proxy.go: log + IncrQueueFull() on queue-full 503 path (was silent)
- proxy.go: remove stub total_tokens/hit_rate from cache-hit log event
- serviceadapter/router.go: Warnf when Auth.Required but jwtValidator nil
- observability: add queue_full_requests_total counter + Prometheus export
- scheduler_test.go: TestBackendAcquireCancelledContext (ctx cancel branch)
- proxy_test.go: implement TestWarmupHeaderHandling (remove t.Skip)
- router_test.go: TestAuthenticate_MethodCapabilityOverrideDirect (auth coverage)

internal/observability/prometheus.go

@@ -36,6 +36,10 @@ func (p *PrometheusExporter) Export() string {
 	lines = append(lines, "# TYPE request_duration_seconds histogram")
 	p.exportRequestDuration(&lines)
 
+	lines = append(lines, "# HELP queue_full_requests_total Requests rejected due to backend queue saturation")
+	lines = append(lines, "# TYPE queue_full_requests_total counter")
+	lines = append(lines, fmt.Sprintf("queue_full_requests_total %d", p.metrics.GetQueueFull()))
+
 	lines = append(lines, "# HELP warmup_requests_delayed_total LLM warmup requests delayed for vLLM slot management")
 	lines = append(lines, "# TYPE warmup_requests_delayed_total counter")
 	lines = append(lines, fmt.Sprintf("warmup_requests_delayed_total %d", p.metrics.GetWarmupDelayed()))