fix: clear expired websocket cookies

[kocaemre]

Summary

• Clear stored WebSocket affinity cookies when a Set-Cookie response explicitly expires them with Max-Age=0 or an epoch Expires value.
• Add regression coverage so reconnects do not keep sending a stale affinity cookie after logout/session rotation.

Test Plan

• uv run pytest tests/http/test_cookies.py tests/http/test_ws_cookies.py -q — 10 passed
• uv run ruff check src/acp/_cookies.py tests/http/test_cookies.py tests/http/test_ws_cookies.py — passed
• uv run ruff format --check src/acp/_cookies.py tests/http/test_cookies.py tests/http/test_ws_cookies.py — 3 files already formatted
• git diff --check — passed
• make test — 258 passed, 1 skipped
• make check — passed