launch_yaml: cache and restore test environment

Several of the launch_yaml tests mutate the os.environ variable, which
can yield some unexpected results depending on the sequence the tests
are run. Use a fixture to cache and restore the original environment.

Signed-off-by: Michael Carroll <carroll.michael@gmail.com>

Author: mjcarroll

launch_yaml/conftest.py

@@ -0,0 +1,28 @@
+# Copyright 2026 Open Source Robotics Foundation, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import os
+import pytest
+
+@pytest.fixture(autouse=True)
+def restore_env():
+    """
+    Restore environment after running tests.
+    
+    Protects the environment when tests mutate or clear it.
+    """
+    original_env = os.environ.copy()
+    yield  # The test runs here
+    os.environ.clear()
+    os.environ.update(original_env)