fix(types): parse steer responses when message type is omitted

[sylvesterkaczmarek]

Summary

The response.steer wire contract added in 3.8.0 allows a user message to omit its optional type field. response.steer.failed can return that original uncommitted input, but the generated response-side steer input model currently requires type and uses it as the discriminator for the message/tool-output union.

As a result, a failed steer event can be valid on the wire but fail SDK validation when the original user message omitted type.

This change:

• makes the stable and beta steer message type optional;
• removes the type discriminator from the two-variant response-side message/tool-output union so an omitted message discriminator can be parsed;
• leaves the tool-output and caller variants unchanged.

Tests

Adds stable and beta regression coverage for response.steer.failed events that return a user message without type.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-03T21:09:09.646543Z	a43566b	PR opened
🔒 Security Review	✅ Completed 2026-09-03T21:12:21.793076Z	a43566b	PR opened

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[sylvesterkaczmarek]

Making type optional requires dropping it as the union discriminator; otherwise valid failed-steer payloads cannot enter the message branch at all. The two branches still have disjoint required fields (role versus tool-output shape), so ordinary union validation remains well constrained. Stable and beta coverage is symmetric.

[sylvesterkaczmarek]

Following up after the Group 6 maintenance pass. This branch is current with main, mergeable, and has no failing checks or unresolved review threads. Could a maintainer review it when convenient?