[pulimsr]

Fixes issues in how the AWS_NEW_RETRIES_2026 opt-in flag is applied:

• With the flag off, the default retry strategy falls back to DefaultRetryStrategy again, as before the flag was added.
• The smithy client reads the flag case-insensitively and keeps it on copy/move.
• With the flag on, AWS_MAX_ATTEMPTS / max_attempts override the DynamoDB default of 4.

Check all that applies:

• 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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