Fixes #4888: Fix spinner persisting in completed tasks UI

- Fixed intermittent state management bug where spinners remained visible on completed task items
- Updated ChatRow component to properly detect when operations have completed:
  - Command execution: Check for api_req_finished, completion_result, or non-partial command_output messages
  - MCP server requests: Check for mcp_server_response messages
  - API requests: Improved logic to properly detect finished/cancelled/failed states
- Added modifiedMessages prop to ChatRow and BrowserSessionRow components for completion detection
- Added comprehensive test suite for spinner logic scenarios
- Ensures spinners are cleared when read/edit activities complete as expected

Author: roomote