fix(sglang): copy caller extra_body before merging structured outputs

[Solaris-star]

Summary

#1931 reported that VLLM/AsyncVLLM._build_client_args mutated the caller's extra_body. That path was fixed in #1933. The same pop + in-place update pattern remains in SGLang / AsyncSGLang._build_client_args, so a reused extra_body dict still leaks structured-output keys (regex / ebnf) into later unconstrained calls.

Change

• Copy the caller extra_body with dict(...) before merging (sync + async)
• Regression test: shared dict is unchanged after a constrained call; unconstrained follow-up has no regex

Test plan

• Pure logic repro of old vs new pop/update behavior
• Added test_sglang_build_client_args_does_not_mutate_caller_extra_body
• Note: package requires Python <3.14; CI on supported versions should run the new test

Fixes #1931

[github-actions]

📚 Documentation preview: https://dottxt-ai.github.io/outlines/pr-preview/pr-1937/

Preview updates automatically with each commit.

[RobinPicard]

Thanks for opening a PR! The change looks good, but the tests are not working because you're using a fixture that does not exist.

[Solaris-star]

Thanks for the review — fixed the test to use the module-level mock openai_client / sglang_model_name globals (same pattern as test_sglang_build_client_args_merges_user_extra_body) instead of non-existent fixtures. Local: 4 passed on the two _build_client_args unit tests.

[Solaris-star]

Re-requesting review: the fixture issue is fixed on the latest commit (module-level openai_client / sglang_model_name globals, same as the existing merge test). CI unit tests for _build_client_args pass locally (4 passed).

[RobinPicard]

Looks good!