test: fix test_main under Python 3.14

Python 3.14 switched multiprocessing to use the "forkserver" start
method. This means the forked subprocess don't inherit the parent
process' memory and the monkeypatched functions (via the `run_taskgraph`
fixture) were no longer applied.

This works around the issue by actually creating bad parameter files to
avoid monkeypatching.

Author: ahal

test/test_main.py

@@ -65,14 +65,23 @@ def test_show_taskgraph_attr(run_taskgraph, capsys, attr, expected):
 
 
 def test_show_taskgraph_parallel(run_taskgraph):
+    # Test that parallel execution works correctly with valid parameters
     res = run_taskgraph(["full", "-p", "taskcluster/test/params"])
     assert res == 0
 
-    # Craft params to cause an exception
-    res = run_taskgraph(
-        ["full", "-p", "taskcluster/test/params"], params={"_kinds": None}
-    )
-    assert res == 1
+
+def test_show_taskgraph_parallel_bad_params(tmp_path):
+    # Create parameter files that will cause processing errors
+    bad_params_dir = tmp_path / "bad_params"
+    bad_params_dir.mkdir()
+    (bad_params_dir / "invalid-yaml.yml").write_text("invalid: yaml: [syntax error")
+
+    try:
+        result = taskgraph_main(["full", "-p", str(bad_params_dir)])
+    except SystemExit as e:
+        result = e.code
+
+    assert result == 1
 
 
 def test_show_taskgraph_force_local_files_changed(mocker, run_taskgraph):