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.

review_redis_env_test.go

@@ -0,0 +1,52 @@
+package traefikoidc
+
+import "testing"
+
+// TestRedisConfig_ApplyEnvFallbacks_Resilience guards the fix that wires the
+// documented REDIS_* resilience env fallbacks (examples/redis-config.yaml) into
+// ApplyEnvFallbacks. Previously an operator following the documented names got
+// no effect: the fields existed but the env vars were never read.
+func TestRedisConfig_ApplyEnvFallbacks_Resilience(t *testing.T) {
+	t.Setenv("REDIS_ENABLED", "true")
+	t.Setenv("REDIS_ENABLE_CIRCUIT_BREAKER", "true")
+	t.Setenv("REDIS_CIRCUIT_BREAKER_THRESHOLD", "7")
+	t.Setenv("REDIS_CIRCUIT_BREAKER_TIMEOUT", "90")
+	t.Setenv("REDIS_ENABLE_HEALTH_CHECK", "true")
+	t.Setenv("REDIS_HEALTH_CHECK_INTERVAL", "45")
+
+	rc := &RedisConfig{}
+	rc.ApplyEnvFallbacks()
+
+	if !rc.Enabled {
+		t.Error("expected Redis enabled from REDIS_ENABLED")
+	}
+	if !rc.EnableCircuitBreaker {
+		t.Error("expected EnableCircuitBreaker from REDIS_ENABLE_CIRCUIT_BREAKER")
+	}
+	if rc.CircuitBreakerThreshold != 7 {
+		t.Errorf("expected CircuitBreakerThreshold 7, got %d", rc.CircuitBreakerThreshold)
+	}
+	if rc.CircuitBreakerTimeout != 90 {
+		t.Errorf("expected CircuitBreakerTimeout 90, got %d", rc.CircuitBreakerTimeout)
+	}
+	if !rc.EnableHealthCheck {
+		t.Error("expected EnableHealthCheck from REDIS_ENABLE_HEALTH_CHECK")
+	}
+	if rc.HealthCheckInterval != 45 {
+		t.Errorf("expected HealthCheckInterval 45, got %d", rc.HealthCheckInterval)
+	}
+}
+
+// TestRedisConfig_ApplyEnvFallbacks_ConfigTakesPrecedence ensures explicitly-set
+// config fields are not overridden by the env fallbacks.
+func TestRedisConfig_ApplyEnvFallbacks_ConfigTakesPrecedence(t *testing.T) {
+	t.Setenv("REDIS_ENABLED", "true")
+	t.Setenv("REDIS_CIRCUIT_BREAKER_THRESHOLD", "9")
+
+	rc := &RedisConfig{Enabled: true, CircuitBreakerThreshold: 4}
+	rc.ApplyEnvFallbacks()
+
+	if rc.CircuitBreakerThreshold != 4 {
+		t.Errorf("expected config value 4 to take precedence, got %d", rc.CircuitBreakerThreshold)
+	}
+}