[releases/v0.18.0][Platform][BugFix] Guard forced tool choice with empty content

[QwertyJack]

What this PR does / why we need it?

This backports the forced-tool-choice content=None guard to the releases/v0.18.0 compatibility layer.

Upstream vLLM still has forced named tool-choice branches that assert content is not None after reasoning extraction. Some reasoning parsers can legally consume the full output and return (reasoning, None), which makes the assert reachable and can surface as a server-side failure.

This PR follows the same compatibility-patch pattern used by:

• 7314bbe2 fix(platform): reimplement MiniMax usage accounting patch (fix(platform): reimplement MiniMax usage accounting patch #7835)
• f83cb0e6 [Bugfix][Platform] Fix GLM47 tool-call finish backfill ([Bugfix][Platform] Fix GLM47 tool-call finish backfill #7710)

The patch is intentionally narrow:

• normalize content=None to "" only for forced named tool choice
• patch both chat-completions and responses parser entry points
• keep the rest of upstream behavior unchanged

Upstream tracking:

• issue: [Bug]: forced tool_choice asserts when reasoning extraction returns content=None vllm#40147
• PR: fix(openai): tolerate empty content in forced tool choice vllm#40148

Does this PR introduce any user-facing change?

Yes.

Forced named tool choice becomes robust when the reasoning parser returns no post-reasoning content, avoiding an internal assertion failure and emitting an empty-argument function call instead.

How was this patch tested?

Unit tests:

pytest -sv tests/ut/patch/platform/test_patch_tool_choice_none_content.py \
  tests/ut/patch/platform/test_patch_glm_tool_call_parser.py \
  tests/ut/patch/platform/test_patch_minimax_usage_accounting.py

Result: 22 passed.

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a compatibility patch for the vLLM platform layer to address an issue where forced named tool choices fail when the reasoning parser returns no content. By normalizing 'None' content to an empty string in specific scenarios, the patch prevents internal assertion errors and ensures robust tool call generation. This change is isolated to the compatibility layer and maintains existing upstream behavior for all other cases.

Highlights

• Forced Tool Choice Guard: Implemented a monkey-patch to normalize 'content=None' to an empty string when forced named tool choice is active, preventing server-side assertion failures.
• Compatibility Layer Update: Applied the fix to both 'OpenAIServing' and 'DelegatingParser' entry points to ensure consistent behavior across different request types.
• Testing: Added comprehensive unit tests to verify that named tool choices are correctly handled even when the reasoning parser returns no content.

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[gemini-code-assist]

Code Review

This pull request introduces a monkey-patch to normalize content=None to an empty string when a forced tool choice is active, preventing server-side failures when reasoning parsers consume all model output. The changes include the patch implementation, documentation updates, and new unit tests. Feedback highlights a Python version compatibility issue regarding the use of the | operator in isinstance() and notes that the PR title and summary need to be updated to comply with the repository's style guide.