fix(test): add environment cleanup for Vertex AI GPT-OSS tests

tests/test_litellm/llms/vertex_ai/vertex_ai_partner_models/gpt_oss/test_vertex_ai_gpt_oss_transformation.py

@@ -16,6 +16,32 @@
 )
 
 
+@pytest.fixture(autouse=True)
+def clean_vertex_env():
+    """Clear Google/Vertex AI environment variables before each test to prevent test isolation issues."""
+    saved_env = {}
+    env_vars_to_clear = [
+        "GOOGLE_APPLICATION_CREDENTIALS",
+        "GOOGLE_CLOUD_PROJECT",
+        "VERTEXAI_PROJECT",
+        "VERTEXAI_LOCATION",
+        "VERTEXAI_CREDENTIALS",
+        "VERTEX_PROJECT",
+        "VERTEX_LOCATION",
+        "VERTEX_AI_PROJECT",
+    ]
+    for var in env_vars_to_clear:
+        if var in os.environ:
+            saved_env[var] = os.environ[var]
+            del os.environ[var]
+
+    yield
+
+    # Restore saved environment variables
+    for var, value in saved_env.items():
+        os.environ[var] = value
+
+
 class TestVertexAIGPTOSSTransformation:
     """Test class for VertexAI GPT-OSS transformation functionality."""