…m providers (#13059)

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## Background

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`amazon-bedrock/anthropic` requires `anthropic_beta` in its request body
to enable extended capabilities for anthropic models, like 1M context
(context-1m-2025-08-07) and fine-grained tool streaming
(fine-grained-tool-streaming-2025-05-14).

## Summary

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- `anthropic`: added `anthropicBeta` and forwarded to downstream
providers via transformRequestBody
- `amazon-bedrock`: receive betas passed from upstream

I also explored handling this gap in `amazon-bedrock` instead (see this
[draft PR](#13054) if you're curious).
Ultimately I think this is the cleanest solution. If we want to allow
both `anthropic` and `bedrock` namespaces, we can handle this on the
Gateway / client side via better education.

## Manual Verification

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- added example scripts

## Checklist

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- [x] Tests have been added / updated (for bug fixes / features)
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fixes / features - run `pnpm changeset` in the project root)
- [x] I have reviewed this pull request (self-review)

## Future Work

- Make these changes in v6 too

## Related Issues