ui: surface agent errors in UI (Claude/Codex/Gemini)

[leeroybrun]

• Problem: Several failure modes were only visible in logs, leading to “nothing happened” in the UI.
• Change:
  • Add a shared helper src/utils/formatErrorForUi.ts for consistent, bounded error formatting.
  • Claude: show Claude process error: <details> instead of a generic “exited unexpectedly”.
  • Codex: surface error conditions directly in UI:
    • streamed MCP error events
    • stream errors (stream_error)
    • MCP startup failures (mcp_startup_update with failed state)
    • tool-call failures when MCP responses return isError for both codex (start) and codex-reply (continue)
  • Gemini: keep existing user-friendly error mapping, but improve generic fallback to include formatted error details.
• Why: Improves debuggability and user trust by ensuring backend/runtime failures aren’t silently hidden.