document and test matched_by on agency-filter diagnostics

[makegov-mark]

What

Tango API 5.9.0 adds matched_by to each entry of meta.resolved_filters.<filter name>[] that resolved, saying how the token matched:

• key: an organization UUID.
• code: a 3-digit CGAC or 4-digit FPDS code.
• name: the organization's name, including a department's everyday name, a spelling variant or a rename.
• alias: an abbreviation or the organization's own alias.
• fuzzy: a looser text match, worth checking against the resolved name.

An entry that did not resolve has no matched_by.
The change is additive.

Why no behaviour change

PaginatedResponse.meta is a plain dict[str, Any] passed through as the API sends it, so matched_by already reaches callers.
This PR only documents the field and pins it with a test.

Changes

• tango/models.py: the meta attribute docstring describes matched_by and its five values.
• README.md: a short example that flags fuzzy matches.
• tests/test_client.py: one test that a response carrying matched_by round-trips through response.meta, and that unresolved_agency_tokens and resolved_agencies are unchanged by it.
• CHANGELOG.md: entry under [Unreleased].

Testing

• uv run ruff format --check tango/
• uv run ruff check tango/
• uv run mypy tango/
• uv run pytest (599 passed, 31 skipped)

Not in this PR

• No version bump; that belongs to the release PR.
• No new accessor. resolved_agencies returns only the matched organizations, so matched_by is read from response.meta directly. An accessor that exposes it (for example one listing loosely matched tokens per filter) would be new public surface and is left as a follow-up decision.

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