Make HTTP cache tests deterministic

[mariuszlipinski]

Summary

• Replace the live W3Schools request with a mocked successful response while preserving cache-write, cache-hit, purge, and re-download assertions.
• Add a deterministic request-pacing test with mocked time, sleep, and HTTP session behavior.
• Keep production code unchanged.

Why

The existing cache test depends on a third-party endpoint that currently returns HTTP 403 to the default client on some machines. Retries make the suite slow, and the result reflects an external access policy rather than py-xbrl cache behavior.

Validation

• ruff check tests/test_cache.py
• ruff format --check tests/test_cache.py
• mypy tests/test_cache.py --check-untyped-defs
• pytest -q tests/test_cache.py — 2 passed
• pytest -q tests/ — 12 passed

[mariuszlipinski]

Superseded by #183. The deterministic test change is preserved there together with the Ruff 0.16 and mypy 2.3 compatibility fixes required for the combined CI matrix to pass.