fix(types): restrict response.steer request inputs - #3793

Summary

The response.steer contract added in 3.8.0 accepts only user messages. Its generated request parameter types currently expose unsupported function_call_output items and server-returned fields (id, status, and beta agent), and require the message type discriminator even though the contract documents it as optional.

This narrows the stable and beta request parameter types to the actual wire contract:

• only user messages are accepted as item-list inputs;
• request messages expose only content, role, and optional type;
• type, when present, remains limited to message.

The response-side models remain unchanged so failed steering events can still deserialize the original rejected input returned by the server.

Tests

Adds regression checks for the stable and beta TypedDict shapes, required keys, and message-only item aliases.