feat(chat): add per-action settlement callback (#4956)

Stream closure alone cannot confirm that a specific action's input was
processed; this adds an optional per-call `onSettled` callback to
`TriggerChatTransport.sendAction()`.

## ✅ Checklist

- [x] I have followed every step in the [contributing
guide](https://github.com/triggerdotdev/trigger.dev/blob/main/CONTRIBUTING.md)
- [x] The PR title follows the convention
- [x] I ran and tested the code works

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## Testing

SDK build, 770 SDK tests (including 45 real-HTTP settlement tests),
ESM/CommonJS package smoke checks, formatting, lint, and Knip passed;
live-worker E2E was not run.

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## Changelog

Adds validated, once-per-call settlement with cancellation and
supersession safeguards, public types, API documentation, and a patch
changeset.
Documents that settlement does not prove application success and that a
missing callback does not make retrying safe.

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## Screenshots

Not applicable — no visual changes

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