## Background


https://docs.aws.amazon.com/bedrock/latest/userguide/service-tiers-inference.html

`@ai-sdk/amazon-bedrock` was missing request-side support for Bedrock
inference service tiers, despite Amazon Bedrock supporting them natively
for `Converse` / `ConverseStream` requests. This created a feature gap
relative to other AI SDK providers such as Groq, which already expose
request-level service tier controls.

Without this, Bedrock users could observe service tier information in
provider metadata, but could not select a tier through
`providerOptions`, even though the underlying Bedrock API supports it.

## Summary

This PR adds request-level Bedrock service tier support via
`providerOptions.bedrock.serviceTier`.

Changes included:
- Added `serviceTier` to `AmazonBedrockLanguageModelOptions`
- Validated supported values: `reserved`, `priority`, `default`, `flex`
- Serialized the option into Bedrock request bodies as top-level
`serviceTier: { type: ... }`
- Added generate and stream tests covering request serialization
- Added schema tests for accepted and rejected values
- Updated the Bedrock provider docs with a usage example
- Added a patch changeset for `@ai-sdk/amazon-bedrock`

## Manual Verification

Manually verified by exercising both `doGenerate` and `doStream` request
construction in the Bedrock provider tests and confirming the outgoing
request body includes:

```ts
serviceTier: {
  type: 'priority',
}
```

Also verified that:
- the value is sent as a top-level Bedrock request field, not under
`additionalModelRequestFields`
- invalid values are rejected by provider option validation
- existing request behavior remains unchanged when `serviceTier` is
omitted

## Checklist

- [x] Tests have been added / updated (for bug fixes / features)
- [x] 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)

## Related Issues

Fixes #13241

---------

Co-authored-by: Aayush Kapoor <83492835+aayush-kapoor@users.noreply.github.com>
Co-authored-by: Aayush Kapoor <aayushkapoor34@gmail.com>