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.

issue67_regression_test.go

@@ -460,11 +460,15 @@ func TestRefreshCoordinatorIntegration(t *testing.T) {
 			return nil, fmt.Errorf("service unavailable")
 		}
 
-		// Trigger circuit breaker
+		// Trigger circuit breaker with genuinely distinct failing operations.
+		// The 100ms dedup delay keeps a rapid same-token call from joining
+		// the previous operation, so sleep so each iteration degrades into
+		// its own refresh operation and records one real failure.
 		for i := 0; i < 4; i++ {
 			ctx := context.Background()
 			_, _ = coordinator.CoordinateRefresh(ctx,
 				fmt.Sprintf("cb_session_%d", i), "refresh_cb", failingRefresh)
+			time.Sleep(150 * time.Millisecond)
 		}
 
 		// Next request should be blocked by circuit breaker