feat: add reasoning event support to Gemini CLI bridge

[ambient-code]

Summary

• Adds ThinkingEvent type to ag_ui_gemini_cli/types.py to parse thinking events from the Gemini CLI NDJSON stream
• Wires up REASONING_START, REASONING_MESSAGE_START/CONTENT/END, and REASONING_END AG-UI event emission in the Gemini CLI adapter, reusing the existing reasoning event models from ag_ui_claude_sdk.reasoning_events
• Properly closes open reasoning blocks on transitions to text messages, tool calls, errors, and run completion
• Adds 5 new tests (2 parse tests + 3 adapter integration tests)

Context

Investigation of the Gemini CLI source confirms that:

• The CLI internally has GeminiEventType.Thought events with ThoughtSummary data (subject + description)
• These are used for the interactive terminal UI (see ThinkingMessage.tsx)
• However, the stream-json output format does not currently emit thinking events — the nonInteractiveCli.ts event loop silently drops them
• The JsonStreamEventType enum only has 6 types: init, message, tool_use, tool_result, error, result

This PR prepares the adapter so it will automatically handle thinking events when the Gemini CLI adds thinking to its streaming JSON output. Until then, unknown event types are harmlessly ignored by parse_event().

Test plan

• All 18 Gemini CLI adapter tests pass (13 existing + 5 new)
• No regressions in the broader runner test suite (15 pre-existing failures unrelated to this change)
• Verify reasoning events render correctly in the frontend once the Gemini CLI adds thinking support upstream

Closes #1127

🤖 Generated with Claude Code

[ambient-code]

Circuit breaker triggered (retry_count=3)

This PR has been retried 3 times without receiving human review. All CI checks pass and there are no merge conflicts or review comments to address.

Current state:

• CI: All checks green
• Conflicts: None
• Review comments: None

The PR is ready for human review. Labeling as ambient-code:needs-human.

[Gkrumbach07]

@ambient-code This is WRONG. you are using claude types in the gemini CLI. You MUST only use Gemini types in the gemini cli and vise versa for claude. research the gemini cli more