feat(mcp): add bounded readiness polling for sources and downloads

[insane66613]

Summary

• add opt-in bounded polling to source_get_content
• add opt-in bounded polling to download_artifact
• keep polling and timeout logic in services/, with thin MCP wrappers
• preserve current behavior by default with wait=False

This is the focused polling behavior discussed during review of #229. It deliberately excludes the ChatGPT bridge, authentication, CDP, and public-route changes from that earlier work.

Behavior

Both tools accept:

• wait: bool = False
• wait_timeout
• poll_interval

Polling uses monotonic deadlines, enforces a minimum interval, and retries only recognized not-ready states. Validation, authentication, permission, and other terminal failures continue to fail immediately.

The download service supports both synchronous and awaitable download implementations without calling asyncio.run() from the MCP wrapper.

Compatibility

• existing calls remain immediate because wait defaults to False
• successful response shapes are unchanged
• no new dependencies

Validation

• uv run ruff check .
• uv run ruff format --check .
• focused source/download service and MCP suites — 164 passed
• dependency composition with structured errors — 207 passed
• full upstream-first integration stack on Windows — 1,318 passed, 39 skipped

A live in-progress NotebookLM source/artifact was not available during final validation, so timeout and transition states were exercised with deterministic service/client tests rather than a live mutation.