[BUG] AWS Bedrock provider requires explicit access key/secret and does not support AWS SDK default credential chain

[mrud]

Describe the bug
The Bedrock provider currently requires bedrockAccessKeyId and bedrockSecretKey in the request payload and in the Agent block UI.

This prevents self-hosted deployments on AWS from using the normal AWS SDK default credential chain, such as:

• EC2 instance profiles
• ECS task roles
• EKS IRSA

Expected behavior
If explicit Bedrock credentials are provided, use them.

If they are not provided, and either AWS_REGION or AWS_PROFILE or any other AWS specific environment variables are set fall back to the AWS SDK default credential chain.