### 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
(#7835)
- `f83cb0e6` [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: vllm-project/vllm#40147
- PR: vllm-project/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:
```bash
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.
---------
Signed-off-by: QwertyJack <7554089+QwertyJack@users.noreply.github.com>
Co-authored-by: QwertyJack <7554089+QwertyJack@users.noreply.github.com>
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