[releases/v0.18.0][Platform][BugFix] Guard forced tool choice with empty content (#8400)

### 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>

vllm_ascend/patch/platform/__init__.py

@@ -30,6 +30,7 @@ import vllm_ascend.patch.platform.patch_sched_yield  # noqa
 import vllm_ascend.patch.platform.patch_torch_accelerator  # noqa
 import vllm_ascend.patch.platform.patch_minimax_usage_accounting  # noqa
 import vllm_ascend.patch.platform.patch_glm_tool_call_parser  # noqa
+import vllm_ascend.patch.platform.patch_tool_choice_none_content  # noqa
 
 if envs.VLLM_ASCEND_BALANCE_SCHEDULING:
     import vllm_ascend.patch.platform.patch_balance_schedule  # noqa