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/dcrstorage/file.go

@@ -83,10 +83,18 @@ func (s *FileStore) Save(ctx context.Context, providerURL string, creds *ClientR
 		return fmt.Errorf("failed to marshal credentials: %w", err)
 	}
 
-	// Write with restrictive permissions (owner read/write only)
-	if err := os.WriteFile(filePath, data, 0600); err != nil {
+	// Write with restrictive permissions (owner read/write only). Write to a
+	// temp file and atomically rename so a crash mid-write can never leave a
+	// truncated credentials file (which would force an avoidable client
+	// re-registration on the next start).
+	tmpPath := filePath + ".tmp"
+	if err := os.WriteFile(tmpPath, data, 0600); err != nil {
 		return fmt.Errorf("failed to write credentials file: %w", err)
 	}
+	if err := os.Rename(tmpPath, filePath); err != nil {
+		_ = os.Remove(tmpPath) // best-effort cleanup
+		return fmt.Errorf("failed to commit credentials file: %w", err)
+	}
 
 	s.logger.Debugf("Saved client credentials to %s", filePath)
 	return nil