fix: reuse existing OAuth client registrations to prevent client_id mismatch

When using auto-discovered OAuth (DCR), LibreChat calls /register on every
flow initiation, getting a new client_id each time. When concurrent
connections or reconnections happen, the client_id used during /authorize
differs from the one used during /token, causing the server to reject the
exchange.

Before registering a new client, check if a valid client registration
already exists in the database and reuse it.

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

packages/api/src/mcp/__tests__/MCPConnectionFactory.test.ts

@@ -270,6 +270,7 @@ describe('MCPConnectionFactory', () => {
         {},
         undefined,
         undefined,
+        oauthOptions.tokenMethods.findToken,
       );
 
       // initFlow must be awaited BEFORE the redirect to guarantee state is stored