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/chat_api.py

@@ -26,6 +26,7 @@ from types import SimpleNamespace
 from quart import Response, request
 
 from api.apps import current_user, login_required
+from api.apps.restful_apis._generation_params import merge_generation_config, pop_generation_config
 from api.db.joint_services.tenant_model_service import (
     get_tenant_default_model_by_type, get_model_config_from_provider_instance, get_api_key, split_model_name
 )
@@ -1173,11 +1174,7 @@ async def session_completion(chat_id_in_arg=""):
     session_id = req.pop("session_id", "") or req.pop("conversation_id", "") or ""
     chat_model_id = req.pop("llm_id", "")
 
-    chat_model_config = {}
-    for model_config in ["temperature", "top_p", "frequency_penalty", "presence_penalty", "max_tokens"]:
-        config = req.get(model_config)
-        if config:
-            chat_model_config[model_config] = config
+    chat_model_config = pop_generation_config(req)
 
     try:
         conv = None
@@ -1220,7 +1217,6 @@ async def session_completion(chat_id_in_arg=""):
                     msg.append(m)
         else:
             dia = _build_default_completion_dialog()
-            dia.llm_setting = chat_model_config
 
         req.pop("messages", None)
         req.pop("question", None)
@@ -1242,6 +1238,7 @@ async def session_completion(chat_id_in_arg=""):
             if not tenant_info or not tenant_info.llm_id:
                 raise LookupError("No default chat model for tenant.")
             dia.llm_id = tenant_info.llm_id
+            merge_generation_config(dia, chat_model_config)
 
         stream_mode = req.pop("stream", True)