test(cron): add _build_job_prompt extra_prompt regression tests

Pins the scheduler-boundary contract: extra_prompt is appended under
'## Run Context', does not mutate job['prompt'], and the header is
absent when extra_prompt is omitted.

Addresses review feedback from harjothkhara on PR #57342.

tests/cron/test_scheduler.py

@@ -1936,6 +1936,30 @@ class TestMultiTargetDeliveryContinuesOnFailure:
         assert "b@example.com" in result
         assert mock_pool.submit.call_count == 2
 
+class TestBuildJobPromptExtraPrompt:
+    """Regression: _build_job_prompt merges extra_prompt into the assembled prompt."""
+
+    def test_extra_prompt_appended_with_header(self):
+        """extra_prompt appears under a '## Run Context' header."""
+        job = {"prompt": "stored prompt"}
+        result = _build_job_prompt(job, extra_prompt="CONTEXT: client=Foo")
+        assert "stored prompt" in result
+        assert "## Run Context" in result
+        assert "CONTEXT: client=Foo" in result
+
+    def test_extra_prompt_does_not_mutate_job(self):
+        """The job dict's 'prompt' field must remain unchanged."""
+        job = {"prompt": "original"}
+        _build_job_prompt(job, extra_prompt="transient context")
+        assert job["prompt"] == "original"
+
+    def test_no_extra_prompt_omits_header(self):
+        """Without extra_prompt, no '## Run Context' header is injected."""
+        job = {"prompt": "just the stored prompt"}
+        result = _build_job_prompt(job)
+        assert "## Run Context" not in result
+        assert "just the stored prompt" in result
+
 
 class TestSetCronSessionTitle:
     """Robust cron session titling: #50535/#50536/#50537."""