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_test.go

@@ -476,11 +476,13 @@ func TestCreateHTTPClient(t *testing.T) {
 
 // TestCreateHTTPClient_Fallback tests fallback when pool is exhausted
 func TestCreateHTTPClient_Fallback(t *testing.T) {
-	// Override global pool with limited one
-	originalPool := globalTransportPool
-	defer func() {
-		globalTransportPool = originalPool
-	}()
+	// Replace the global pool with a full one. resetGlobalTransportPoolForTest
+	// is required: the global is sync.Once-guarded, so overriding the var
+	// alone would be clobbered by GetTransportPool's once-body, and restoring
+	// a nil original after the Once was consumed would leave later callers
+	// panicking on a nil receiver.
+	resetGlobalTransportPoolForTest()
+	defer resetGlobalTransportPoolForTest()
 
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
@@ -504,9 +506,17 @@ func TestCreateHTTPClient_Fallback(t *testing.T) {
 		return
 	}
 
+	// When the pool is exhausted, GetTransport returns nil and CreateHTTPClient
+	// falls back to a basic client: timeout set, no transport, no redirect policy.
 	if client.Timeout != timeout {
 		t.Errorf("Client timeout should be %v, got %v", timeout, client.Timeout)
 	}
+	if client.Transport != nil {
+		t.Error("Fallback client should have no transport")
+	}
+	if client.CheckRedirect != nil {
+		t.Error("Fallback client should have no redirect policy")
+	}
 }
 
 // TestTransportPool_ConcurrentAccess tests concurrent access to transport pool