test: add unit test for vertex_ai embedding extra_headers

tests/test_litellm/test_vertex_ai_embedding_headers.py

@@ -0,0 +1,16 @@
+import pytest
+from litellm import embedding
+
+def test_vertex_ai_embedding_extra_headers():
+    # Test that extra_headers are passed without crashing
+    try:
+        response = embedding(
+            model="vertex_ai/text-embedding-004",
+            input=["hello"],
+            extra_headers={"X-Custom-Header": "test-value"}
+        )
+    except Exception as e:
+        # We expect a 401/404 if no real creds, 
+        # but we are checking that it doesn't fail with a TypeError
+        if "extra_headers" in str(e):
+            pytest.fail("extra_headers not accepted by vertex_ai embedding")