### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### What happened?

When starting an OAuth flow for an MCP server, the temporary server created for the OAuth process does not inherit the OAuth URLs (`authorization_url`, `token_url`, `registration_url`) from the existing saved server. This causes the OAuth flow to fail with "Authorization URL not set" error.

### Root Cause

In `litellm/proxy/management_endpoints/mcp_management_endpoints.py`, the `_inherit_credentials_from_existing_server` function only inherits `credentials` from the existing server, but not the OAuth URLs.

Current code (~line 130): ```python async def _inherit_credentials_from_existing_server( payload: MCPServerPayload, ) -> MCPServerPayload: # ... fetches existing server ... if existing_server.credentials: updates["credentials"] = existing_server.credentials # OAuth URLs are NOT inherited ```

### Expected Behavior

When creating a temporary MCP server for OAuth flow, it should inherit all OAuth-related fields from the existing saved server: - `authorization_url` - `token_url` - `registration_url`

### Steps to Reprod…