fix(mcp): flag a partial available_authorities list in list_alerts (closes #24)

[dchaudhari7177]

Mirrors #9's fix onto the MCP surface, as the issue describes.

What changed

ListAlertsResult gains available_authorities_partial, populated from the same response.partial /alerts uses, set on the line right after the list it qualifies so the two surfaces cannot drift apart again.

Two decisions worth flagging

None, not False, when there is no list. available_authorities is only built for an unknown authority, so most responses have nothing to qualify. A bare false there would read as "the authority list is complete" — a claim about a list that was never built. The field is bool | None and tracks the list's own presence. Third test covers this.

The description too. As the issue says, that text is what a model reads when deciding how to interpret the result, so it now names the field and says what to do with it:

Check available_authorities_partial before treating that list as complete: when it is true a source was down, so an authority absent from the list may still be real and may still have alerts. Do not report an authority as unknown or as having no alerts on the strength of a partial list.

The last sentence is deliberately imperative about the paraphrase rather than the JSON, since the paraphrase is where the false negative actually lands.

Tests

Four, using the FakeAdapter partial-fetch pattern from tests/test_mcp_server.py:

• The real case: us-nws asked for, its source timed out, so it cannot appear in available_authorities — the response now says the list is partial rather than implying the authority does not exist.
• The flag can say no: every source answered, so a missing authority really is absent.
• The flag is absent when the list is.
• The description names the field, read off the registered tool rather than the source string.

Red before green — stashing only src/alertmux/mcp_server.py and re-running:

FAILED test_unknown_authority_flags_a_partial_available_authorities_list
FAILED test_available_authorities_partial_is_false_on_a_complete_fetch
FAILED test_available_authorities_partial_is_absent_when_the_list_is
FAILED test_list_alerts_description_tells_the_model_to_check_the_flag
4 failed, 8 passed

Exactly the four new tests, nothing else.

Full suite: 435 passed, 2 failed. Both failures — test_gdacs.py::test_geometry_falls_back_to_bbox_polygon_when_no_point and test_tsunami.py::test_naive_updated_is_quarantined_rather_than_assuming_utc — reproduce on pristine main at 1ef609b with this branch's two files stashed, so they are the local baseline and not from this change.

One setup note: the tests need the mcp extra, and mcp>=2,<3 — the mcp.server.mcpserver.MCPServer import fails against 1.x with ModuleNotFoundError: No module named 'mcp.server.mcpserver', which is easy to misread as a path problem. pip install -e ".[dev,mcp]" in a clean venv is what makes tests/test_mcp_server.py collect at all.

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