FEATURE: Add support for credential_source=Environment in SharedConfig

[ducks]

Implement missing credential_source = Environment support in Ruby SDK

Problem

The AWS CLI documentation clearly states that credential_source supports three values:

• Environment – Specifies that the AWS CLI is to retrieve source credentials from environment variables
• Ec2InstanceMetadata – Specifies that the AWS CLI is to use the IAM role attached to the EC2 instance profile
• EcsContainer – Specifies that the AWS CLI is to use the IAM role attached to the ECS container

However, the Ruby SDK only implemented the latter two, causing InvalidCredentialSourceError when users try to use the documented Environment option.

Solution

This adds the missing Environment case in SharedConfig#credentials_from_source that retrieves credentials from:

• AWS_ACCESS_KEY_ID
• AWS_SECRET_ACCESS_KEY
• AWS_SESSION_TOKEN

This matches the documented AWS CLI behavior.

Testing

• Added comprehensive test coverage for the new Environment credential source
• All existing gems tests pass (19620 examples, 0 failures)
• Verified no regressions across AWS SDK ecosystem

Impact

Fixes a gap where documented AWS credential functionality throws InvalidCredentialSourceError in the Ruby SDK.

[jterapin]

Hi! Thank you for your contribution. I will be taking a look.

[richardwang1124]

Hey, thanks for opening a PR. I agree that we should support this and I may make some changes to this PR as well.

[ducks]

cool cool. thanks. I'm open to any feedback/help. seemed like too simple a fix but those are the best kinds I guess.

[jterapin]

Nice! Thanks y'all.

[ducks]

wow, thanks to y'all as well for helping me get this merged in so fast.

One question I had was on this comment:

I actually think we don't need to bump s3 min core since this credential_source usage only applies within Amazon EC2 instances or Amazon Elastic Container Service containers

I am trying to use this on a self-hosted, non ec2 metal server and was seeing the same error when trying to use credential_source = Environment.

I see the other 2 options for the case are for ec2 instances but it seems like this should work elsewhere fwict.

[richardwang1124]

You're correct, credential_source = Environment is not specific to EC2/ECS, it should work everywhere. Thanks again for your contribution! The changes should be going out as part of today's release.