Surface AskBill sources in search_documentation MCP tool

[phoenixy1]

Summary

• The search_documentation MCP tool was discarding the sources array returned by the AskBill websocket and only forwarding answer.
• Without citations, hallucinated URLs in the answer text slip through unchallenged — I hit this in practice while researching Plaid trial plans and got a plausible-but-broken plaid.com/docs/account/plans/#trial URL with no way to verify.
• Append a ## Sources section to the response, formatted from the SourceMetadata shape (title, url) defined in plaid/finn-mvp:message_types.py. URLs are deduplicated; sources with neither URL nor title are skipped.
• Title text is escaped (\, [, ]) and URLs use the <url> autolink form so titles like Auth [v2] and URLs containing parens (e.g. Plaid_(company)) render correctly. The answer is rstrip()ed before the Sources section to avoid stacked blank lines.

Drive-by: pytest config fix

CI was failing on a pre-existing latent bug in sandbox/pyproject.toml: [tool.pytest] should be [tool.pytest.ini_options], and python_files must be a list, not a bare string. Older pytest tolerated both; pytest 9 hard-fails:

TypeError: pyproject.toml: config option 'python_files' expects a list for type 'args', got str: 'test_*.py'

Last green CI was Oct 2025 (pytest 8.x); this PR was the first run since pytest 9 entered the environment, so it tripped here. Fix is in its own commit (Fix pytest config in pyproject.toml) so it's reviewable on its own — happy to split into a separate PR if preferred.

Verification

Ran the local checkout end-to-end against the real AskBill websocket (Claude Code MCP config swapped from uvx mcp-server-plaid to uv run --directory <local-clone> mcp-server-plaid):

• Same query that previously hallucinated plaid.com/docs/account/plans/#trial now appends a ## Sources section with four real support.plaid.com/hc/en-us/articles/... URLs returned by AskBill's retrieval — confirming the prior link was an LLM hallucination with no source backing it.
• Sources render as clickable markdown autolinks; no markdown breakage observed.
• Existing 21 unit tests pass locally with the pytest config fix.

Test plan

• Manually invoke the tool from an MCP client and confirm the ## Sources section appears with clickable links
• Existing test suite passes locally
• Confirm a query that produces no sources still returns just the answer (no empty Sources section) — relies on AskBill returning []; not yet observed in practice

🤖 Generated with Claude Code