Return tool errors for invalid web fetch URLs

[MohamedAkbarally]

Summary

• Return a failed ToolResult when fetch_web_page receives a non-absolute web URL, such as a local filesystem path.
• Keep the existing httpx fetch path unchanged for absolute http:// and https:// URLs.
• Add focused tests for local-path rejection and unchanged valid URL fetching.

Backward compatibility

This is backward compatible because the new guard only intercepts inputs that httpx cannot fetch successfully with the existing default transport: URLs without an http/https scheme or without a network location. Any absolute HTTP or HTTPS URL still flows through the same _fetch implementation, headers, retries, and response handling as before. Previously crashing inputs now return success=False to the model instead of aborting the agent run.

Test plan

• uv run ruff format src/stirrup/tools/web.py tests/test_web_search.py
• uv run pytest tests/test_web_search.py
• uv run ruff check src/stirrup/tools/web.py tests/test_web_search.py

Made with Cursor

[declanjackson]

Looks good, just one small comment on the error msg