fix(cron): thread per-run prompt through cronjob(action='run') (#57331)

Salvaged from PR #57342 by @liuhao1024 (with the injection-scan half
from PR #57360 by @ghedeselmabot): cronjob(action='run', prompt=...)
silently discarded the prompt argument — per-run context never
reached the spawned cron session.

The prompt is now threaded as extra_prompt through the whole chain
(cronjob run action → _try_dispatch_background_run/_execute_job_now →
_run_claimed_job → run_one_job → run_job → _build_job_prompt) and
appended to the stored prompt under a '## Run Context' header for
that single fire only — never persisted to the job definition. It
passes the same strict _scan_cron_prompt injection scan as stored
prompts before firing, and works identically on the background and
sync fallback paths.

Test fakes across tests/cron/ updated to accept the new kwargs
(sibling-test blast radius from the signature change).

Co-authored-by: liuhao1024 <liuhao1024@users.noreply.github.com>

tests/cron/test_scheduler.py

@@ -1268,7 +1268,7 @@ class TestParallelTick:
         barrier = threading.Barrier(2, timeout=5)
         call_order = []
 
-        def mock_run_job(job, *, defer_agent_teardown=None):
+        def mock_run_job(job, *, defer_agent_teardown=None, **kw):
             """Each job hits a barrier — both must be active simultaneously."""
             call_order.append(("start", job["id"]))
             barrier.wait()  # blocks until both threads reach here
@@ -1302,7 +1302,7 @@ class TestParallelTick:
         from gateway.session_context import get_session_env
         seen = {}
 
-        def mock_run_job(job, *, defer_agent_teardown=None):
+        def mock_run_job(job, *, defer_agent_teardown=None, **kw):
             origin = job.get("origin", {})
             # run_job sets ContextVars — verify each job sees its own
             from gateway.session_context import set_session_vars, clear_session_vars