Fixes 12 CI test failures:
1. test_cli_new_session (4): _FakeAgent missing commit_memory_session
attribute added in the memory provider refactoring. Added MagicMock.
2. test_run_progress_topics (1): already_sent detection only checked
stream consumer flags, missing the response_previewed path from
interim_assistant_callback. Restructured guard to check both paths.
3. test_timezone (1): HERMES_TIMEZONE leaked into child processes via
_SAFE_ENV_PREFIXES matching HERMES_*. The code correctly converts
it to TZ but didn't remove the original. Added child_env.pop().
4. test_session_env (1): contextvars baseline captured from a different
context couldn't be restored after clear. Changed assertion to verify
the test's value was removed rather than comparing to a fragile baseline.
5. test_discord_slash_commands (5): already fixed on current main.
e4cd62d07d