aws_credentials: make IRSA authoritative in credentials chain

[edsiper]

Fixes #11255

• Added explicit constants for IRSA environment variables to clarify detection of pod-level AWS authentication settings.

• Updated the standard AWS credential chain to treat IRSA as authoritative, failing fast if the EKS provider cannot initialize and skipping ECS/EC2 fallbacks when IRSA is configured.

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✨ Finishing touches

🧪 Generate unit tests (beta)

• [ ] Create PR with unit tests
• [ ] Post copyable unit tests in a comment
• [ ] Commit unit tests in branch aws-irsa-issue

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[edsiper]

@codex review

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Codex Review: Didn't find any major issues. Bravo.

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