fix(process): label background completion causes (#46659)

Track why a background process finished and include that source in notify-on-complete messages so SIGTERM from process.kill, kill_all, backend loss, and ordinary exits are distinguishable.

tests/tools/test_notify_on_complete.py

@@ -12,7 +12,7 @@ import json
 import os
 import time
 import pytest
-from unittest.mock import patch
+from unittest.mock import MagicMock, patch
 
 from tools.process_registry import (
     ProcessRegistry,
@@ -99,6 +99,8 @@ class TestCompletionQueue:
         assert completion["session_id"] == s.id
         assert completion["command"] == "echo hello"
         assert completion["exit_code"] == 0
+        assert completion["completion_reason"] == "exited"
+        assert completion["termination_source"] == ""
         assert "build succeeded" in completion["output"]
 
     def test_move_to_finished_nonzero_exit(self, registry):
@@ -138,6 +140,35 @@ class TestCompletionQueue:
         completion = registry.completion_queue.get_nowait()
         assert completion["exit_code"] == -15  # from the first (kill) call
 
+    def test_kill_process_sets_completion_reason_and_source(self, registry):
+        s = _make_session(notify_on_complete=True, output="stopping")
+        s.process = MagicMock()
+        s.process.pid = 4242
+        registry._running[s.id] = s
+
+        class FakeProcess:
+            def __init__(self, pid):
+                self.pid = pid
+
+            def children(self, recursive=False):
+                return []
+
+            def terminate(self):
+                pass
+
+        import psutil as _psutil
+
+        with patch.object(_psutil, "Process", side_effect=lambda pid: FakeProcess(pid)), \
+             patch.object(registry, "_write_checkpoint"):
+            result = registry.kill_process(s.id)
+
+        assert result["status"] == "killed"
+        assert result["completion_reason"] == "killed"
+        assert result["termination_source"] == "process.kill"
+        completion = registry.completion_queue.get_nowait()
+        assert completion["completion_reason"] == "killed"
+        assert completion["termination_source"] == "process.kill"
+
     def test_output_truncated_to_2000(self, registry):
         """Long output is truncated to last 2000 chars."""
         long_output = "x" * 5000