chore(litellm): validate /v1/models response with shared schema in initialize()

Match the API route handler — both code paths now run the same
vllmUpstreamResponseSchema.parse() over the upstream /v1/models
JSON instead of a raw type-cast, so malformed upstream payloads
surface a descriptive ZodError instead of a downstream TypeError.

Addresses Greptile review feedback on PR #4739.

Author: waleedlatif1

apps/sim/providers/litellm/index.ts

@@ -67,7 +67,8 @@ export const litellmProvider: ProviderConfig = {
         return
       }
 
-      const data = (await response.json()) as { data: Array<{ id: string }> }
+      const { vllmUpstreamResponseSchema } = await import('@/lib/api/contracts/providers')
+      const data = vllmUpstreamResponseSchema.parse(await response.json())
       const models = data.data.map((model) => `litellm/${model.id}`)
 
       this.models = models