fix(cron): mark session as ended after job completes (#2998)

Cron was the only execution path that never called end_session(),
leaving ended_at = NULL permanently. This made cron sessions invisible
to hermes prune --older-than and indistinguishable from active sessions.

Captures session_id in a local variable before agent construction so
it's available in the finally block even if AIAgent() fails, then calls
end_session(session_id, 'cron_complete') before close().

Cherry-picked from PR #2979 by ygd58. Fixed bug: original PR called
end_session() with zero arguments (TypeError — method requires
session_id and end_reason).

Fixes #2972.

Co-authored-by: ygd58 <ygd58@users.noreply.github.com>

tests/cron/test_scheduler.py

@@ -254,6 +254,10 @@ class TestRunJobSessionPersistence:
         assert kwargs["session_db"] is fake_db
         assert kwargs["platform"] == "cron"
         assert kwargs["session_id"].startswith("cron_test-job_")
+        fake_db.end_session.assert_called_once()
+        call_args = fake_db.end_session.call_args
+        assert call_args[0][0].startswith("cron_test-job_")
+        assert call_args[0][1] == "cron_complete"
         fake_db.close.assert_called_once()
 
     def test_run_job_empty_response_returns_empty_not_placeholder(self, tmp_path):