feat!: Support invoke with structured output in VercelAI provider

[jsonbailey]

Note

Add structured output invocation to VercelProvider and harden error handling with corresponding tests.

• VercelProvider (packages/ai-providers/server-ai-vercel/src/VercelProvider.ts):
  • Add invokeStructuredModel(messages, responseStructure) using generateObject + jsonSchema; returns StructuredResponse with metrics and raw JSON.
  • Wrap invokeModel in try/catch; log warnings and return failure metrics on errors; preserve success path with usage metrics.
• Tests (__tests__/VercelProvider.test.ts):
  • Add tests for structured invocation success, no-usage, and error cases; mock generateObject/jsonSchema.
  • Add error-path test for invokeModel; clear mocks in beforeEach.

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