fix(google-auth): add AWS Fargate metadata endpoint support

[ggrossman]

Summary

On AWS Fargate, the IMDS security credentials endpoint does not use role names in the URL structure (unlike EC2 instances). The current _DefaultAwsSecurityCredentialsSupplier unconditionally fetches the IAM role name and appends it to the credentials URL, which fails on Fargate with a RefreshError.

This PR:

• Adds _is_fargate_environment() to detect Fargate via ECS-specific environment variables (ECS_CONTAINER_METADATA_URI_V4, ECS_CONTAINER_METADATA_URI, or AWS_EXECUTION_ENV containing AWS_ECS_FARGATE)
• Skips the role name lookup on Fargate and calls the security credentials URL directly
• Preserves the existing EC2 behavior (3 metadata requests: region, role, credentials) when not on Fargate
• Fargate path makes only 2 metadata requests: region + credentials

Behavior comparison

Environment	Metadata requests	Credentials URL
EC2 (unchanged)	region → role name → credentials	{url}/{role_name}
Fargate (new)	region → credentials	{url} (no role suffix)

Files changed

• packages/google-auth/google/auth/aws.py — Fargate detection + conditional role name handling
• packages/google-auth/tests/test_aws.py — 5 new test cases covering all detection methods, EC2 regression, and unit tests for _is_fargate_environment()

Fixes https://github.com/googleapis/google-auth-library-python/issues/1099

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[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves an issue where Google Auth's AWS security credentials supplier failed on AWS Fargate due to an incorrect assumption about the metadata endpoint URL structure. It introduces logic to detect Fargate environments and adjust the credential retrieval process to bypass the role name lookup, aligning with Fargate's metadata service behavior. This change ensures seamless authentication for applications running on Fargate while maintaining compatibility with traditional EC2 environments.

Highlights

• Fargate Environment Detection: Implemented _is_fargate_environment() to identify AWS Fargate based on ECS environment variables.
• Conditional Role Name Handling: Modified credential retrieval to skip IAM role name lookup for Fargate environments, as their metadata endpoint does not use role names in the URL.
• Optimized Metadata Requests: Reduced the number of metadata requests for Fargate environments from three (region, role, credentials) to two (region, credentials).
• EC2 Behavior Preservation: Ensured that the existing behavior for EC2 instances, including role name lookup, remains unchanged.
• Comprehensive Testing: Added five new test cases to cover Fargate detection methods, Fargate credential retrieval, and to confirm EC2 regression.

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[gemini-code-assist]

Code Review

This pull request adds support for AWS Fargate metadata endpoints by detecting the Fargate environment and skipping the IAM role name lookup, which is not used in Fargate's credential URL structure. The changes are well-implemented and include comprehensive tests for the new logic and for ensuring existing EC2 behavior is preserved. The suggestion to improve code conciseness is valid and has been retained.

[gemini-code-assist]

For improved readability and conciseness, you can combine these checks into a single boolean expression.

        return bool(
            os.environ.get("ECS_CONTAINER_METADATA_URI_V4")
            or os.environ.get("ECS_CONTAINER_METADATA_URI")
            or "AWS_ECS_FARGATE" in os.environ.get("AWS_EXECUTION_ENV", "")
        )