Audit review (rounds 1-31): fix confirmed issues, remove dead code, harden tests

Security/correctness fixes:
- fail-open token validation preserved with lenient-audience; access token that
  fails verification and has no ID token to corroborate now fails closed
- token_manager blacklist-expiry: fix impossible tautology (x>24h && x<24h)
- constant-time OAuth state compare; clear state/nonce/code_verifier on
  callback token-exchange failure to prevent replay of half-failed flows
- StopAllTasks gated on last-instance close
- background-task panic recovery; remove task-registry counter leak
- circuit breaker half-open: release probe slot; CB + per-session outcome
  recorded once per refresh operation, not once per waiter
- Redis connection pool: handle nil receive when pool closed concurrently
- FileStore: atomic temp-write + rename
- errors_recovery health-check data race -> atomic

Test-isolation / hardening:
- reset helpers for Once-pinned global cache manager and transport pool
- coverage: utils 70->93% (redirect URL security contract); fuzz targets for
  bearer parsers; concurrent ServeHTTP race tests for the shared request path

Dead-code removal (verified orphaned, build+suite green): internal/providers
(30 files, 0 importers), internal/compat, internal/features, input_validation.

Verified: govulncheck, go mod tidy, yaegi-validate, cross-compile (darwin+
linux), golangci-lint (new-from-rev clean), 3x full race+shuffle green.

internal/pool/transport.go

@@ -60,21 +60,37 @@ var (
 
 // GetTransportPool returns the global transport pool instance
 func GetTransportPool() *TransportPool {
-	transportPoolOnce.Do(func() {
-		ctx, cancel := context.WithCancel(context.Background())
-		globalTransportPool = &TransportPool{
-			transports:  make(map[string]*sharedTransport),
-			maxConns:    20,
-			ctx:         ctx,
-			cancel:      cancel,
-			clientCount: 0,
-			maxClients:  5,
-		}
-		go globalTransportPool.cleanupRoutine(ctx)
-	})
+	if globalTransportPool == nil {
+		transportPoolOnce.Do(func() {
+			ctx, cancel := context.WithCancel(context.Background())
+			globalTransportPool = &TransportPool{
+				transports:  make(map[string]*sharedTransport),
+				maxConns:    20,
+				ctx:         ctx,
+				cancel:      cancel,
+				clientCount: 0,
+				maxClients:  5,
+			}
+			go globalTransportPool.cleanupRoutine(ctx)
+		})
+	}
 	return globalTransportPool
 }
 
+// resetGlobalTransportPoolForTest clears the process-global transport pool so the
+// next call to GetTransportPool returns a fresh instance, and cancels the
+// current pool's cleanup goroutine. Required for order-independent tests that
+// replace the global pool: the singleton is sync.Once-guarded, so without a
+// reset a consumed Once leaves GetTransportPool returning nil and later callers
+// panic on a nil receiver (see TestCreateHTTPClient_Fallback).
+func resetGlobalTransportPoolForTest() {
+	if globalTransportPool != nil && globalTransportPool.cancel != nil {
+		globalTransportPool.cancel()
+	}
+	transportPoolOnce = sync.Once{}
+	globalTransportPool = nil
+}
+
 // DefaultTransportConfig returns a secure default configuration
 func DefaultTransportConfig() TransportConfig {
 	return TransportConfig{