test(proxy): add regression tests for vertex passthrough model names with slashes (#19855)

Added test cases for custom model names containing slashes in Vertex AI
passthrough URLs (e.g., gcp/google/gemini-2.5-flash).

Test cases:
- gcp/google/gemini-2.5-flash
- gcp/google/gemini-3-flash-preview
- custom/model

tests/local_testing/test_auth_utils.py

@@ -356,6 +356,25 @@ def test_get_internal_user_header_from_mapping_no_internal_returns_none():
             "/openai/deployments/my-deployment/chat/completions",
             "my-deployment"
         ),
+        # Custom model_name with slashes (e.g., gcp/google/gemini-2.5-flash)
+        # This is the NVIDIA P0 bug fix - regex should capture full model name including slashes
+        (
+            {},
+            "/vertex_ai/v1/projects/my-project/locations/us-central1/publishers/google/models/gcp/google/gemini-2.5-flash:generateContent",
+            "gcp/google/gemini-2.5-flash"
+        ),
+        # Another custom model_name with slashes
+        (
+            {},
+            "/vertex_ai/v1/projects/my-project/locations/global/publishers/google/models/gcp/google/gemini-3-flash-preview:generateContent",
+            "gcp/google/gemini-3-flash-preview"
+        ),
+        # Model name with single slash
+        (
+            {},
+            "/vertex_ai/v1/projects/my-project/locations/us-central1/publishers/google/models/custom/model:generateContent",
+            "custom/model"
+        ),
     ],
 )
 def test_get_model_from_request_vertex_ai_passthrough(request_data, route, expected_model):