test(parser): add unit tests for payload_fixer module

[vishnuprakaz]

Description

Add unit tests for the payload_fixer module in the Python agent SDK.

The parse_and_fix function had no dedicated unit tests — it was only
exercised indirectly via conformance tests. This PR adds direct coverage
for all behaviors in the module.

Are these changes tested?

Yes — 20 tests covering:

• Valid JSON list and single-object normalization
• Smart quote (curly quote) normalization
• Trailing comma removal (objects, arrays, nested structures)
• Multiple messages in a single payload
• Logging warnings on autofix
• Invalid payloads raising ValueError with the correct message

Testing/Running Instructions

1. Navigate to the Python SDK directory: cd agent_sdks/python
2. Run the new tests: uv run pytest tests/parser/test_payload_fixer.py -v
3. Verify all 20 tests pass.

Pre-launch Checklist

• I signed the [CLA].
• I read the [Contributors Guide].
• I read the [Style Guide].
• I have added updates to the [CHANGELOG].
• I updated/added relevant documentation.
• My code changes (if any) have tests.
• If my branch is on fork, I have verified that scripts/e2e_test.sh passes.

[gemini-code-assist]

Code Review

This pull request introduces a new test suite for the parse_and_fix utility, covering JSON normalization, trailing comma autofixes, and error handling. The review feedback highlights opportunities to improve test coverage by including single smart quotes and primitive JSON types (e.g., booleans, nulls) as invalid payloads. The reviewer also pointed out several redundant test cases that could be removed to reduce code duplication and improve clarity.

[ditman]

/cc @nan-yu do you mind taking a look at these tests?

[nan-yu]

We have migrated unit tests into conformance tests so that it can be shared across multiple SDK languages: https://github.com/google/A2UI/blob/main/agent_sdks/conformance/suites/parser.yaml#L83.

Are there any missing test cases? If so, we should add them to the conformance test spec.

[vishnuprakaz]

Thanks @nan-yu! Looking at suites/parser.yaml, the basic fix_payload cases are already covered. A few scenarios from this PR aren't in the YAML yet nested trailing commas, multiple messages, and additional invalid payload shapes. Should I add those to parser.yaml and close this PR?

[nan-yu]

Thanks @nan-yu! Looking at suites/parser.yaml, the basic fix_payload cases are already covered. A few scenarios from this PR aren't in the YAML yet nested trailing commas, multiple messages, and additional invalid payload shapes. Should I add those to parser.yaml and close this PR?

You can go either way, either opening a new PR or repurposing this PR with new conformance tests in parser.yaml.