Fix: Chat completion generation parameter overrides (#15389)

### What problem does this PR solve?

Closes #15388.

Chat completion routes did not reliably honor per-request generation
settings:

- `/api/v1/chat/completions` copied generation settings with a
truthiness check, so valid zero values such as `temperature: 0`, `top_p:
0`, `frequency_penalty: 0`, `presence_penalty: 0`, and `max_tokens: 0`
were dropped.
- `/api/v1/openai/{chat_id}/chat/completions` did not forward standard
generation settings into the request-specific dialog LLM settings before
calling `async_chat`.

This PR preserves explicitly supplied generation parameters, including
zero values, and merges request-level overrides into existing dialog
settings where appropriate.

The supported generation parameter keys and merge behavior live in a
shared REST API helper to keep both completion routes aligned.

Validation:

- `git diff --check`
- `python3 -m py_compile api/apps/restful_apis/_generation_params.py
api/apps/restful_apis/chat_api.py api/apps/restful_apis/openai_api.py
test/testcases/test_http_api/test_session_management/test_session_sdk_routes_unit.py`
- `uv run ruff check api/apps/restful_apis/_generation_params.py
api/apps/restful_apis/chat_api.py api/apps/restful_apis/openai_api.py
test/testcases/test_http_api/test_session_management/test_session_sdk_routes_unit.py`
- `ZHIPU_AI_API_KEY=dummy uv run pytest
test/testcases/test_http_api/test_session_management/test_session_sdk_routes_unit.py
-q -k generation_params`

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

api/apps/restful_apis/_generation_params.py

@@ -0,0 +1,38 @@
+#
+#  Copyright 2026 The InfiniFlow Authors. All Rights Reserved.
+#
+#  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.
+#
+
+from copy import deepcopy
+
+GENERATION_CONFIG_KEYS = ("temperature", "top_p", "frequency_penalty", "presence_penalty", "max_tokens")
+
+
+def extract_generation_config(req):
+    return {key: req[key] for key in GENERATION_CONFIG_KEYS if key in req and req[key] is not None}
+
+
+def pop_generation_config(req):
+    generation_config = extract_generation_config(req)
+    for key in GENERATION_CONFIG_KEYS:
+        req.pop(key, None)
+    return generation_config
+
+
+def merge_generation_config(dialog, generation_config):
+    if not generation_config:
+        return
+    llm_setting = deepcopy(getattr(dialog, "llm_setting", None) or {})
+    llm_setting.update(generation_config)
+    dialog.llm_setting = llm_setting