Bugfix: Race Condition During Agent Switching

This commit fixes the race condition bug where switching from mock agent to real agent (claude/gemini) caused no model response until ESC was pressed.

Root Cause

When switching agents, both the OLD and NEW agents share the same app_event_tx channel. The OLD agent may still be processing/shutting down when the NEW agent starts up, causing events from the OLD agent to leak into the NEW widget before SessionConfigured was received.

Fix

Added event filtering mechanism to ChatWidget:

• expected_model: Option<String> in ChatWidgetInit specifies which model the widget expects
• session_configured_received: bool tracks whether SessionConfigured has arrived
• handle_codex_event() filters events when expected_model is set:
  • All events ignored until SessionConfigured arrives
  • SessionConfigured only accepted if model name matches (case-insensitive)
  • Once matching event arrives, normal event processing resumes

Changes

• tui/src/chatwidget.rs - Core event filtering logic
• tui/src/app.rs - Pass expected_model during agent switch
• tui/src/app_backtrack.rs - Set expected_model: None for backtrack
• tui-pty-e2e/tests/agent_switching.rs - New E2E tests for event isolation
• Cleanup: Removed unused ClearPendingAgent event and has_pending_agent method
• Fixed clippy warnings for dead code

Testing

• Added test_agent_switch_message_flow_mock_to_mock_alt E2E test
• Added test_agent_switch_logs_correct_sequence E2E test
• All 14 agent_switching tests pass
• Full workspace clippy check passes