[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

vllm_ascend/patch/platform/patch_tool_choice_none_content.py

@@ -0,0 +1,86 @@
+#
+# Copyright (c) 2026 Huawei Technologies Co., Ltd. All Rights Reserved.
+# This file is a part of the vllm-ascend project.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+# OpenAI forced tool choice: tolerate None content after reasoning extraction.
+#
+
+from __future__ import annotations
+
+from openai.types.responses import ToolChoiceFunction
+from vllm.entrypoints.openai.chat_completion.protocol import (
+    ChatCompletionNamedToolChoiceParam,
+)
+from vllm.entrypoints.openai.engine.serving import OpenAIServing
+from vllm.parser.abstract_parser import DelegatingParser
+
+
+def _normalize_tool_choice_content(
+    request,
+    content: str | None,
+) -> str | None:
+    if content is not None:
+        return content
+
+    tool_choice = getattr(request, "tool_choice", None)
+    if isinstance(
+        tool_choice,
+        (ToolChoiceFunction, ChatCompletionNamedToolChoiceParam),
+    ):
+        return ""
+    return content
+
+
+_original_parse_tool_calls_from_content = OpenAIServing._parse_tool_calls_from_content
+
+
+def _patched_parse_tool_calls_from_content(
+    request,
+    tokenizer,
+    enable_auto_tools: bool,
+    tool_parser_cls,
+    content: str | None = None,
+):
+    content = _normalize_tool_choice_content(request, content)
+    return _original_parse_tool_calls_from_content(
+        request=request,
+        tokenizer=tokenizer,
+        enable_auto_tools=enable_auto_tools,
+        tool_parser_cls=tool_parser_cls,
+        content=content,
+    )
+
+
+OpenAIServing._parse_tool_calls_from_content = staticmethod(_patched_parse_tool_calls_from_content)
+
+_original_delegating_parse_tool_calls = DelegatingParser._parse_tool_calls
+
+
+def _patched_delegating_parse_tool_calls(
+    self,
+    request,
+    content: str | None,
+    enable_auto_tools: bool,
+):
+    content = _normalize_tool_choice_content(request, content)
+    return _original_delegating_parse_tool_calls(
+        self,
+        request,
+        content,
+        enable_auto_tools,
+    )
+
+
+DelegatingParser._parse_tool_calls = _patched_delegating_parse_tool_calls