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.

refresh_coordinator_test.go

@@ -215,7 +215,9 @@ func TestCircuitBreakerProtection(t *testing.T) {
 		return nil, fmt.Errorf("service unavailable")
 	}
 
-	// Cause circuit breaker to trip
+	// Cause circuit breaker to trip with genuinely distinct failing
+	// operations (sleep so each same-token call is not absorbed as a join
+	// on the still-in-flight previous operation).
 	var tripCount int
 	for i := 0; i < 5; i++ {
 		ctx := context.Background()
@@ -225,6 +227,7 @@ func TestCircuitBreakerProtection(t *testing.T) {
 			"refresh_token",
 			refreshFunc,
 		)
+		time.Sleep(150 * time.Millisecond)
 
 		if err != nil && err.Error() == "refresh circuit breaker is open due to repeated failures" {
 			tripCount++