feat(cron): track delivery failures in job status (#6042)

_deliver_result() now returns Optional[str] — None on success, error
message on failure. All failure paths (unknown platform, platform
disabled, config load error, send failure, unresolvable target)
return descriptive error strings.

mark_job_run() gains delivery_error param, tracked as
last_delivery_error on the job — separate from agent execution errors.
A job where the agent succeeded but delivery failed shows
last_status='ok' + last_delivery_error='...'.

The cronjob list tool now surfaces last_delivery_error so agents and
users can see when cron outputs aren't arriving.

Inspired by PR #5863 (oxngon) — reimplemented with proper wiring.

Tests: 3 new mark_job_run tests + 6 new _deliver_result return tests.

tests/cron/test_jobs.py

@@ -339,6 +339,36 @@ class TestMarkJobRun:
         assert updated["last_status"] == "error"
         assert updated["last_error"] == "timeout"
 
+    def test_delivery_error_tracked_separately(self, tmp_cron_dir):
+        """Agent succeeds but delivery fails — both tracked independently."""
+        job = create_job(prompt="Report", schedule="every 1h")
+        mark_job_run(job["id"], success=True, delivery_error="platform 'telegram' not configured")
+        updated = get_job(job["id"])
+        assert updated["last_status"] == "ok"
+        assert updated["last_error"] is None
+        assert updated["last_delivery_error"] == "platform 'telegram' not configured"
+
+    def test_delivery_error_cleared_on_success(self, tmp_cron_dir):
+        """Successful delivery clears the previous delivery error."""
+        job = create_job(prompt="Report", schedule="every 1h")
+        mark_job_run(job["id"], success=True, delivery_error="network timeout")
+        updated = get_job(job["id"])
+        assert updated["last_delivery_error"] == "network timeout"
+        # Next run delivers successfully
+        mark_job_run(job["id"], success=True, delivery_error=None)
+        updated = get_job(job["id"])
+        assert updated["last_delivery_error"] is None
+
+    def test_both_agent_and_delivery_error(self, tmp_cron_dir):
+        """Agent fails AND delivery fails — both errors recorded."""
+        job = create_job(prompt="Report", schedule="every 1h")
+        mark_job_run(job["id"], success=False, error="model timeout",
+                     delivery_error="platform 'discord' not enabled")
+        updated = get_job(job["id"])
+        assert updated["last_status"] == "error"
+        assert updated["last_error"] == "model timeout"
+        assert updated["last_delivery_error"] == "platform 'discord' not enabled"
+
 
 class TestAdvanceNextRun:
     """Tests for advance_next_run() — crash-safety for recurring jobs."""

tests/cron/test_scheduler.py

@@ -508,6 +508,90 @@ class TestDeliverResultWrapping:
         assert send_mock.call_args.kwargs["thread_id"] == "17585"
 
 
+class TestDeliverResultErrorReturns:
+    """Verify _deliver_result returns error strings on failure, None on success."""
+
+    def test_returns_none_on_successful_delivery(self):
+        from gateway.config import Platform
+
+        pconfig = MagicMock()
+        pconfig.enabled = True
+        mock_cfg = MagicMock()
+        mock_cfg.platforms = {Platform.TELEGRAM: pconfig}
+
+        with patch("gateway.config.load_gateway_config", return_value=mock_cfg), \
+             patch("tools.send_message_tool._send_to_platform", new=AsyncMock(return_value={"success": True})):
+            job = {
+                "id": "ok-job",
+                "deliver": "origin",
+                "origin": {"platform": "telegram", "chat_id": "123"},
+            }
+            result = _deliver_result(job, "Output.")
+        assert result is None
+
+    def test_returns_none_for_local_delivery(self):
+        """local-only jobs don't deliver — not a failure."""
+        job = {"id": "local-job", "deliver": "local"}
+        result = _deliver_result(job, "Output.")
+        assert result is None
+
+    def test_returns_error_for_unknown_platform(self):
+        job = {
+            "id": "bad-platform",
+            "deliver": "origin",
+            "origin": {"platform": "fax", "chat_id": "123"},
+        }
+        with patch("gateway.config.load_gateway_config"):
+            result = _deliver_result(job, "Output.")
+        assert result is not None
+        assert "unknown platform" in result
+
+    def test_returns_error_when_platform_disabled(self):
+        from gateway.config import Platform
+
+        pconfig = MagicMock()
+        pconfig.enabled = False
+        mock_cfg = MagicMock()
+        mock_cfg.platforms = {Platform.TELEGRAM: pconfig}
+
+        with patch("gateway.config.load_gateway_config", return_value=mock_cfg):
+            job = {
+                "id": "disabled",
+                "deliver": "origin",
+                "origin": {"platform": "telegram", "chat_id": "123"},
+            }
+            result = _deliver_result(job, "Output.")
+        assert result is not None
+        assert "not configured" in result
+
+    def test_returns_error_on_send_failure(self):
+        from gateway.config import Platform
+
+        pconfig = MagicMock()
+        pconfig.enabled = True
+        mock_cfg = MagicMock()
+        mock_cfg.platforms = {Platform.TELEGRAM: pconfig}
+
+        with patch("gateway.config.load_gateway_config", return_value=mock_cfg), \
+             patch("tools.send_message_tool._send_to_platform", new=AsyncMock(return_value={"error": "rate limited"})):
+            job = {
+                "id": "rate-limited",
+                "deliver": "origin",
+                "origin": {"platform": "telegram", "chat_id": "123"},
+            }
+            result = _deliver_result(job, "Output.")
+        assert result is not None
+        assert "rate limited" in result
+
+    def test_returns_error_for_unresolved_target(self, monkeypatch):
+        """Non-local delivery with no resolvable target should return an error."""
+        monkeypatch.delenv("TELEGRAM_HOME_CHANNEL", raising=False)
+        job = {"id": "no-target", "deliver": "telegram"}
+        result = _deliver_result(job, "Output.")
+        assert result is not None
+        assert "no delivery target" in result
+
+
 class TestRunJobSessionPersistence:
     def test_run_job_passes_session_db_and_cron_platform(self, tmp_path):
         job = {