🔌 fix: Resolve MCP OAuth flow state race condition (#11941)

* 🔌 fix: Resolve MCP OAuth flow state race condition

The OAuth callback arrives before the flow state is stored because
`createFlow()` returns a long-running Promise that only resolves on
flow COMPLETION, not when the initial PENDING state is persisted.
Calling it fire-and-forget with `.catch(() => {})` meant the redirect
happened before the state existed, causing "Flow state not found"
errors.

Changes:
- Add `initFlow()` to FlowStateManager that stores PENDING state and
  returns immediately, decoupling state persistence from monitoring
- Await `initFlow()` before emitting the OAuth redirect so the
  callback always finds existing state
- Keep `createFlow()` in the background for monitoring, but log
  warnings instead of silently swallowing errors
- Increase FLOWS cache TTL from 3 minutes to 10 minutes to give
  users more time to complete OAuth consent screens

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* 🔌 refactor: Revert FLOWS cache TTL change

The race condition fix (initFlow) is sufficient on its own.
TTL configurability should be a separate enhancement via
librechat.yaml mcpSettings rather than a hardcoded increase.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* 🔌 fix: Address PR review — restore FLOWS TTL, fix blocking-path race, clean up dead args

- Restore FLOWS cache TTL to 10 minutes (was silently dropped back to 3)
- Add initFlow before oauthStart in blocking handleOAuthRequired path
  to guarantee state persistence before any redirect
- Pass {} to createFlow metadata arg (dead after initFlow writes state)
- Downgrade background monitor .catch from logger.warn to logger.debug
- Replace process.nextTick with Promise.resolve in test (correct semantics)
- Add initFlow TTL assertion test
- Add blocking-path ordering test (initFlow → oauthStart → createFlow)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

packages/api/src/flow/manager.ts

@@ -88,6 +88,24 @@ export class FlowStateManager<T = unknown> {
     return normalizedExpiresAt < Date.now();
   }
 
+  /**
+   * Stores initial PENDING flow state without starting the monitor loop.
+   * Use this when you need to guarantee the state is persisted before
+   * performing an action (e.g., an OAuth redirect), then call createFlow()
+   * separately to start monitoring for completion.
+   */
+  async initFlow(flowId: string, type: string, metadata: FlowMetadata = {}): Promise<void> {
+    const flowKey = this.getFlowKey(flowId, type);
+    const initialState: FlowState = {
+      type,
+      status: 'PENDING',
+      metadata,
+      createdAt: Date.now(),
+    };
+    logger.debug(`[${flowKey}] Storing initial flow state`);
+    await this.keyv.set(flowKey, initialState, this.ttl);
+  }
+
   /**
    * Creates a new flow and waits for its completion
    */