Session view should surface API rate-limit and session errors

[benjaminshafii]

Problem\nWhen a session fails due to API rate limiting or other session-related errors, the Session view does not show the error details in the UI.\n\n## Current behavior\n- Session encounters an API rate-limit or related session error\n- Failure is not clearly surfaced in Session view\n- Users cannot tell why the session failed\n\n## Expected behavior\nBubble up relevant error messages in Session view so end users can understand what happened and what to do next.\n\n## Impact\nLack of visible error context makes failures confusing and slows recovery/troubleshooting for users.