fix: invalidate favicon cache when clearing watch history

[snowyukitty]

Summary

• Evict the module-level favicon filename cache after clear_watch() removes watch history files.
• Add a regression test that primes the cache, clears history, and verifies the deleted favicon is no longer returned.

Fixes #4277.

Thanks @ebarkhordar for the thorough root-cause analysis and reproduction in #4277.

Testing

• Regression test fails on upstream master with 'favicon.ico' is not None, then passes with this fix.
• pytest -q tests/unit/test_watch_model.py — 19 passed
• pytest -q tests/unit/ tests/llm/ — 340 passed, 30 subtests passed
• ruff check . --select E9,F63,F7,F82,INT
• git diff --check

[dgtlmoon]

LGTM :)