fix(core): prefer websocket with request hooks - #43234

Summary

• add narrow transport-neutral Session model request hooks for endpoint and header changes
• scope model-aware hooks by provider so GitHub Copilot HTTP middleware does not disable OpenAI WebSockets
• preserve ChatGPT OAuth headers and Codex endpoint rewriting across HTTP and WebSocket
• verify WebSocket selection with the built-in OpenAI and GitHub Copilot hooks enabled
• keep the experimental WebSocket flag disabled by default

Testing

• Plugin typecheck and build
• Core typecheck and build
• Provider, Promise adapter, and Session generation tests
• WebSocket selection and HTTP-hook policy tests
• 28 Session WebSocket manager and local-server tests
• Workspace typecheck