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Description of changes:

• Implement Retry Behavior 2.1 core logic gated behind AWS_NEW_RETRIES_2026
• Fixes adaptive retry strategy to match other SDKs

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• Did a review by yourself.
• Added proper tests to cover this PR. (If tests are not applicable, explain.)
• Checked if this PR is a breaking (APIs have been changed) change.
• Checked if this PR will not introduce cross-platform inconsistent behavior.
• Checked if this PR would require a ReadMe/Wiki update.

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