fix(test): add environment cleanup for Vertex AI Qwen tests

Add autouse pytest fixture to clear Google/Vertex AI environment
variables before each test, preventing authentication errors in CI.

Previous tests may set GOOGLE_APPLICATION_CREDENTIALS or other Vertex
environment variables and not clean them up, causing this test to
attempt real Google authentication instead of using mocks.

This fix:
- Adds clean_vertex_env fixture with autouse=True
- Saves and clears Google/Vertex env vars before each test
- Restores them after each test
- Prevents "AuthenticationError: Request had invalid authentication
  credentials" (401) in CI when run with other tests

Same fix pattern as PR #21268 (rerank) and PR #21272 (GPT-OSS).

Related: test was failing on PR #21217, but NOT caused by PR #21217
(which only modifies test_anthropic_structured_output.py). This is
another test isolation issue.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

tests/test_litellm/llms/vertex_ai/vertex_ai_partner_models/qwen/test_vertex_ai_qwen_global_endpoint.py

@@ -24,6 +24,30 @@ from litellm.llms.vertex_ai.vertex_llm_base import VertexBase
 from litellm.types.llms.vertex_ai import VertexPartnerProvider
 
 
+@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",
+        "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 TestQwenGlobalOnlyDetection:
     """Test that Qwen models are correctly identified as global-only."""