Fix GPT-OSS streaming tool call parsing

[QwertyJack]

Thanks for your contribution and we appreciate it a lot. The following instructions would make your pull request more healthy and more easily receiving feedbacks. If you do not understand some items, don't worry, just make the pull request and seek help from maintainers.

Motivation

Fix incorrect function-call payloads when GPT-OSS runs in streaming mode. The aim is to ensure tool invocations receive clean JSON arguments and to guard against regressions with a dedicated test.

Fix #4014

Modification

Updated the GPT-OSS streaming parser so tool-call arguments aren’t duplicated across chunks and added a regression unit test that exercises the streaming behaviour with a stubbed harmony parser.

BC-breaking (Optional)

No

Use cases (Optional)

No

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit tests to ensure the correctness.
3. If the modification has a dependency on downstream projects of a newer version, this PR should be tested with all supported versions of downstream projects.
4. The documentation has been modified accordingly, like docstring or example tutorials.