Fix manual cron session rotation

[fallintoplace]

Summary

Fixes the manual cron session rotation endpoint so it actually clears a persistent job's SDK session context.

Root cause

Manual rotation called _maybe_rotate_context(..., rotate_hours=0). That used to force rotation, but the context_rotate_at refactor made rotate_hours=0 mean "disabled" inside the shared helper, so the manual path returned rotated: false without clearing sdk_session_id.

Changes

• Add an explicit force flag to the shared rotation helper.
• Use force=True from rotate_session().
• Add a regression test covering manual rotation when scheduled rotation is disabled.

Validation

• python -m pytest tests/test_cron.py -q
• python -m pytest tests/test_cron.py::TestRotationMemorize -q