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.

tools/code_execution_tool.py

@@ -1016,10 +1016,13 @@ def execute_code(
         _existing_pp = child_env.get("PYTHONPATH", "")
         child_env["PYTHONPATH"] = _hermes_root + (os.pathsep + _existing_pp if _existing_pp else "")
         # Inject user's configured timezone so datetime.now() in sandboxed
-        # code reflects the correct wall-clock time.
+        # code reflects the correct wall-clock time.  Only TZ is set —
+        # HERMES_TIMEZONE is an internal Hermes setting and must not leak
+        # into child processes.
         _tz_name = os.getenv("HERMES_TIMEZONE", "").strip()
         if _tz_name:
             child_env["TZ"] = _tz_name
+        child_env.pop("HERMES_TIMEZONE", None)
 
         # Per-profile HOME isolation: redirect system tool configs into
         # {HERMES_HOME}/home/ when that directory exists.