fix(tests): resolve remaining CI failures — commit_memory_session, already_sent, timezone leak, session env (#10785)

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.

tests/cli/test_cli_new_session.py

@@ -34,6 +34,7 @@ class _FakeAgent:
             [{"id": "t1", "content": "unfinished task", "status": "in_progress"}]
         )
         self.flush_memories = MagicMock()
+        self.commit_memory_session = MagicMock()
         self._invalidate_system_prompt = MagicMock()
 
         # Token counters (non-zero to verify reset)