test(companion): Add transactional reread test

Add the missing transactionality assertion to the existing reread coverage: a
batch that would change one companion and add another but also contains a
duplicate name is rejected as a whole. The process set and configs stay
untouched and no fork or kill happens, proving nothing is applied on validation
failure.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/design/companion-process-manager.md

@@ -698,7 +698,7 @@ No per-companion logic in Arbiter.
 - [x] Add tests for config validation.
 - [x] Add tests for state transitions.
 - [x] Add tests for control commands.
-- [ ] Add tests for transactional reread.
+- [x] Add tests for transactional reread.
 - [ ] Add tests that HTTP worker behavior is unchanged.
 
 ## 21. Test Plan

tests/test_companion_manager.py

@@ -285,6 +285,23 @@ def test_reread_duplicate_name_keeps_old():
     assert "duplicate" in result["error"]
 
 
+def test_reread_validation_failure_mutates_nothing():
+    manager = make_manager("rq")
+    original_config = manager.processes["rq"].config
+    original_names = set(manager.processes)
+    # This batch would change "rq" and add "scheduler", but the duplicate
+    # "scheduler" makes the whole reread invalid: nothing must be applied.
+    bad = [make_config("rq", env={"X": "1"}), make_config("scheduler"),
+           make_config("scheduler")]
+    with mock.patch("os.fork") as fork, mock.patch("os.kill") as kill:
+        result = manager.reread_config(bad)
+    assert result["ok"] is False and result["kept_old_config"] is True
+    assert set(manager.processes) == original_names
+    assert manager.processes["rq"].config is original_config
+    fork.assert_not_called()
+    kill.assert_not_called()
+
+
 def test_handle_command_reread_no_loader():
     manager = make_manager("rq")
     with pytest.raises(CommandError):