[fix](s3)Use anonymous credentials for S3-compatible storage when credentials are absent

[CalvinKirs]

…

For S3-compatible object storages such as COS and OBS, authentication does not fully follow the AWS native credential provider chain. In these systems, anonymous access is a valid and commonly used mode when no AK/SK is configured.

However, when aws_credentials_provider_version is set to v2, BE currently falls back to the AWS SDK v2 default credential provider chain if no credentials are explicitly provided. This behavior is AWS-specific and is not applicable to S3-compatible storage systems like COS and OBS, which may not support or require the AWS credential resolution chain.

Problem

When aws_credentials_provider_version = v2 and no AK/SK is configured:

• BE attempts to resolve credentials using the AWS SDK v2 provider chain
• This may lead to unexpected authentication failures or unnecessary environment dependency
• The behavior is inconsistent with S3-compatible storage expectations

Expected Behavior

For S3-compatible object storage:

• If AK/SK is explicitly provided, use the configured credentials
• If no credentials are provided, fall back to anonymous credentials
• Do NOT trigger the AWS SDK v2 default credential provider chain

[hello-stephen]

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