Add comprehensive httpmock-based unit tests for the client package covering:
- List: success path, discussions disabled, limit/filter validation, pagination
- Search: success path, filter validation, pagination
- ListCategories: success path, discussions disabled
Tests use httpmock.Registry with defer Verify(t) to ensure all stubs
are exercised, following the established testing pattern in this repo.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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