docs(model-adapter): fix extra-params tooltip wording (merge→override)

ModelAdapterModal.vue described OpenAI extra params as "合并到" (merge into)
the request body, but the backend (request_override.go:applyExtraParams)
writes body[name] = value — same-name fields are overridden, not merged.
This contradicted ModelEditor.vue's tooltip ("覆盖到...同名字段以这里为准")
and risked users expecting array fields like `tools` to be appended,
which would silently replace the built-in function tools.

Align the modal tooltip with the editor view and the actual override
semantics.

frontend/src/components/ModelAdapterModal.vue

@@ -40,7 +40,7 @@ const openAIEndpointOptions = [
 ];
 
 const fieldTips = {
-  openAIExtraParams: "开启后会把 JSON 对象合并到 OpenAI 请求体。OpenAI service_tier 支持 auto、default、flex、scale、priority；priority 可用于高优先级/Fast 类场景。",
+  openAIExtraParams: "开启后会把 JSON 对象覆盖到 OpenAI 请求体。同名字段以这里为准。OpenAI service_tier 支持 auto、default、flex、scale、priority；priority 可用于高优先级/Fast 类场景。",
 };
 
 const props = defineProps({