--register-aws does not detect AWS Bedrock API credentials — users get false sense of security

## Bug description

`git-secrets --register-aws` registers patterns for classic AWS credential types (AKIA, ASIA, AGPA, etc.) but does not cover **AWS Bedrock API keys**, which use a distinct short-lived credential format.

A developer using Bedrock who runs `git-secrets --register-aws` will see no warnings when committing Bedrock credentials, creating a false sense of security.

PR #271 (open since October 2025) attempts to add a Bedrock pattern but has not been merged. This issue documents the gap so it is tracked independently of that PR.

## To reproduce

1. Install git-secrets and run `git-secrets --register-aws`
2. Create a file containing a Bedrock short-lived credential
3. Attempt to commit — git-secrets does not block it

## Expected behavior

`--register-aws` should cover all active AWS credential types, including Bedrock API keys, or the documentation should explicitly state which credential types are **not** covered so users know to add patterns manually.

## Environment

- git-secrets version: latest (master)
- OS: Linux / macOS

## References

- PR #271 adds a partial fix but has been awaiting review since October 2025

Start at the --register-aws entry point and inspect PR #271 to understand the existing Bedrock pattern attempt. Reproduce the missing warning with a Bedrock short-lived credential, then verify that the chosen fix detects it or that the documentation clearly states the unsupported credential types.

Written by the indexing model from the issue text.