fix: Limit Bedrock GPT models to default service tier (#25318)

## Description

Bedrock currently only supports the implicit `default` service tier for
GPT models. This PR strips non-default service tier metadata from
Bedrock model catalogs so Codex does not advertise or send unsupported
tiers.

## What changed

- Normalize both built-in and configured Bedrock catalogs to
default-only service tier behavior.
- Add regression coverage for built-in and configured Bedrock catalogs.

## Validation

- `just fmt`
- `just test -p codex-model-provider`