Bedrock 连接应使用本机 AWS 凭证，而不是通用 API Key

来自 #4402 的 P2：Amazon Bedrock 预设走 AWS SDK 的 IAM/配置文件凭证链，没有可填进通用 API Key 框的密钥。向导把缺省鉴权当成 API Key，没填就不能继续，配置正确的 AWS 用户只能编造一个字符串才能保存。

本 PR 不新做一套 AWS 登录。后续应给 Bedrock 单独建模宿主 AWS 凭证，并做可用性检查。

Start by tracing the Bedrock preset through the setup wizard and its generic API Key validation. Check how AWS SDK IAM or profile credentials are represented and how the wizard decides whether authentication is complete. Done means a correctly configured AWS user can save Bedrock without inventing an API key, with a Bedrock-specific availability check.

Written by the indexing model from the issue text.