Set ChecksumMode for S3 response checksum validation

[stobrien89]

Description

Sets the modeled response checksum validation member to ENABLED when response_checksum_validation is when_supported and the operation supports a checksum algorithm available to the SDK.

Because ApplyChecksumMiddleware runs after serialization, this updates both the command member and its modeled request header.

This change also:

• Preserves explicitly configured validation modes.
• Leaves ChecksumMode unset for when_required unless explicitly enabled.
• Prevents checksum control headers such as x-amz-checksum-mode from suppressing request checksum calculation.
• Preserves existing presigned-request behavior.

Testing

• Added middleware tests for default, explicit, required, and unsupported-algorithm behavior.
• Added coverage for checksum control-header detection.
• Added full client-stack and presigned-request coverage.

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