[Bugfix] Replace assert with ValueError for response_format validation in completions endpoint (#35456)

Signed-off-by: umut-polat <52835619+umut-polat@users.noreply.github.com>

tests/entrypoints/openai/test_completion_error.py

@@ -221,6 +221,19 @@ async def test_completion_error_stream():
     assert chunks[-1] == "data: [DONE]\n\n"
 
 
+def test_json_schema_response_format_missing_schema():
+    """When response_format type is 'json_schema' but the json_schema field
+    is not provided, request construction should raise a validation error
+    so the API returns 400 instead of 500."""
+    with pytest.raises(Exception, match="json_schema.*must be provided"):
+        CompletionRequest(
+            model=MODEL_NAME,
+            prompt="Test prompt",
+            max_tokens=10,
+            response_format={"type": "json_schema"},
+        )
+
+
 def test_negative_prompt_token_ids_nested():
     """Negative token IDs in prompt (nested list) should raise validation error."""
     with pytest.raises(Exception, match="greater than or equal to 0"):