fix(agent): Autofix trailing commas in LLM-generated JSON

[selamw1]

Fixes json.JSONDecodeError caused by LLMs returning JSON with trailing commas.

Changes

• Added a regex pre-processing step in RestaurantAgent to strip trailing commas before calling json.loads().
• Improves parsing robustness and prevents unnecessary retries.

Fixes

Resolves #480

[gemini-code-assist]

Code Review

This pull request aims to fix JSON decoding errors from LLM-generated content by removing trailing commas. The approach of using a regular expression is sound. However, the implemented regex is not fully robust and can fail in cases of multiple trailing commas. I've suggested a more robust regex that handles these cases correctly.

[gemini-code-assist]

The current regex only handles a single trailing comma. It would fail to fix a string with multiple trailing commas, like [1, 2,,], which would be converted to the still-invalid [1, 2,]. Using a positive lookahead is more robust and handles multiple trailing commas correctly in a single pass.

Suggested change

	json_string_cleaned = re.sub(r",\s*([\]}])", r"\1", json_string_cleaned)
	json_string_cleaned = re.sub(r",(?=\s*[\]}])", "", json_string_cleaned)

[wrenj]

thanks for this

can you put it in the SDK here:
https://github.com/google/A2UI/blob/main/a2a_agents/python/a2ui_agent/src/a2ui/extension/send_a2ui_to_client_toolset.py#L265

and add some tests

I think flow would be
if not jsonschema.validate():
run fixers
if trailing comma fixer does something
emit warning

jsonschema.validate

[selamw1]

Thanks, I've adjusted the code into the SDK and followed the suggested flow. I also added unit tests to verify the fix and the warning message.