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/cache/backends/redis_pool.go

@@ -83,6 +83,11 @@ func (p *ConnectionPool) Get(ctx context.Context) (*RedisConn, error) {
 
 		select {
 		case conn = <-p.connections:
+			if conn == nil {
+				// Pool channel was closed concurrently (Close) - receiving from a
+				// closed channel yields the zero value.
+				return nil, ErrBackendClosed
+			}
 			// Reuse existing connection - validate if health check enabled
 			if p.config.EnableHealthCheck && !p.isConnectionHealthy(conn) {
 				// Connection is stale, close it and try again
@@ -118,6 +123,11 @@ func (p *ConnectionPool) Get(ctx context.Context) (*RedisConn, error) {
 			// Pool exhausted, wait for a connection with timeout
 			select {
 			case conn = <-p.connections:
+				if conn == nil {
+					// Pool channel was closed concurrently (Close) - receiving from a
+					// closed channel yields the zero value.
+					return nil, ErrBackendClosed
+				}
 				// Validate connection if health check enabled
 				if p.config.EnableHealthCheck && !p.isConnectionHealthy(conn) {
 					_ = conn.Close()