fix(togetherai): enable structured outputs for DeepSeek V4 Flash (#19836)

## Background

Together lists `deepseek-ai/DeepSeek-V4-Flash-0731` as supporting
structured outputs in its [serverless model
catalog](https://docs.together.ai/docs/serverless/models), and its
[structured output
guide](https://docs.together.ai/docs/inference/chat/structured-outputs)
accepts JSON Schema through `response_format`.

`@ai-sdk/togetherai` currently leaves `supportsStructuredOutputs`
disabled. The OpenAI-compatible layer therefore rewrites `json_schema`
requests to `json_object`, discarding the schema, name, description, and
strict setting with an unsupported-format warning.

## Summary

- Enable structured outputs for the exact documented DeepSeek V4 Flash
model.
- Keep unknown and unsupported Together models disabled.
- Add regression coverage for both the capability decision and
constructor wiring.
- Update the generate and stream object examples to use DeepSeek V4
Flash with shape-explicit JSON prompts.
- Add the model ID to typed suggestions and include a patch changeset.

## End-to-End Verification

Manually exercised the patched Together provider request path for both
generation and streaming with an enum-constrained schema. Both outgoing
requests contained `response_format.type: json_schema`, the full schema,
name, description, and `strict: true`, with no unsupported-format
warning. An unknown model retained the existing `json_object` fallback
and warning.

A live native Together request was not run because this environment does
not have a `TOGETHER_API_KEY`.

## Checklist

- [x] All commits are signed (PRs with unsigned commits cannot be
merged)
- [x] Tests have been added / updated (for bug fixes / features)
- [ ] Documentation has been added / updated (for bug fixes / features)
- [x] A _patch_ changeset for relevant packages has been added (for bug
fixes / features - run `pnpm changeset` in the project root)
- [x] I have reviewed this pull request (self-review)