refactor(ai): make OpenAI Responses extend Open Responses - #38681

Summary

• add a provider-neutral Open Responses protocol, options namespace, and metadata namespace
• compose OpenAI Responses over the baseline for image generation, hosted tools, event aliases, WebSocket transport, and OpenAI defaults
• move compatible and Vertex Responses routes to the baseline without inheriting OpenAI behavior

Testing

• bun test in packages/ai (434 passed, 28 skipped)
• bun typecheck in packages/ai
• bun typecheck in packages/core
• bun test test/model-resolver.test.ts test/aisdk.test.ts in packages/core
• repository-wide bun typecheck via the pre-push hook

Closes #38537