feat: Unify the 'Add Model Provider' modal (#15768)

### What problem does this PR solve?

feat：Unify the 'Add Model Provider' modal

### Type of change

- [x] New Feature (non-breaking change which adds functionality)
- [x] Refactoring

web/src/services/llm-service.ts

@@ -8,6 +8,7 @@ const {
   addProvider,
   addProviderInstance,
   verifyProviderConnection,
+  listProviderModels,
   listProviderInstances,
   listInstanceModels,
   showProviderInstance,
@@ -45,6 +46,10 @@ const methods = {
     url: verifyProviderConnection,
     method: 'post',
   },
+  listProviderModels: {
+    url: listProviderModels,
+    method: 'get',
+  },
   listProviderInstances: {
     url: listProviderInstances,
     method: 'get',