feat(ai): add Vertex Responses entrypoint

[nexxeln]

summary

• add a Vertex Responses provider route that composes the reusable OpenAI Responses protocol for Grok models
• expose @opencode-ai/ai/providers/google-vertex/responses with Vertex OAuth, explicit access tokens, lazy ADC, and store: false by default
• cover the official endpoint, request and stream shapes, protocol identity, package loading, auth constraints, and update provider parity docs

testing

• bun test in packages/ai (330 passed, 29 skipped)
• bun typecheck and bun run build in packages/ai
• bun typecheck and bun test test/session-runner-model.test.ts in packages/core
• repository typecheck via push hook