[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/__init__.py

@@ -238,6 +238,26 @@
 #       finish-backfill fix are present in the runtime vLLM version used by
 #       vllm-ascend.
 #
+# ** 11. File: platform/patch_tool_choice_none_content.py**
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+#   1. `vllm.entrypoints.openai.engine.serving.OpenAIServing`
+#      `vllm.parser.abstract_parser.DelegatingParser`
+#    Why:
+#       Some reasoning parsers can consume the full model output and return
+#       `content=None`. On the release runtime, forced named tool choice still
+#       asserts that content is present before constructing a function call,
+#       which can surface as a server-side failure instead of an empty-argument
+#       tool call.
+#    How：
+#       Monkey-patch the forced-tool-choice parsing entry points to normalize
+#       `content=None` to `""` before delegating back to the original upstream
+#       implementations.
+#    Related PR (if no, explain why):
+#       https://github.com/vllm-project/vllm/pull/40148
+#    Future Plan:
+#       Remove this patch once the upstream forced-tool-choice fix is included
+#       in the runtime vLLM version used by vllm-ascend.
+#
 # * Worker Patch:
 # ===============
 #