Problem: Kattis and USACO had zero offline test coverage — no fixtures,
no conftest routers, and no entries in the test matrix. Precision, error
cases, and the Kattis contest-vs-slug fallback were also untested.
Solution: Add HTML fixtures for both platforms covering metadata, tests,
and contest list modes. Wire up conftest routers that patch
`httpx.AsyncClient.get` using the same pattern as CSES/CodeChef. Extend
the test matrix to include Kattis and USACO (18 parametrized cases, up
from 12). Add a dedicated test for the Kattis contest-path (verifies
`contest_url`/`standings_url` are set). Add parametrized metadata error
tests for CSES, USACO, and Kattis. Assert `precision` field type in all
tests-mode payloads; `usaco/problem_1471.html` includes an absolute-error
hint to exercise `extract_precision`.
