[v2] Fix bug with response_checksum_validation not being respected when using CRT

[aemous]

Description of changes:

• Updated vendored s3transfer so that response_checksum_validation is respected when using CRT client to perform S3 downloads.
• Added a regression test to cover this new behavior.

Description of tests:

• Ran and passed all tests (see GitHub Actions).
• Verified that regression test fails without the fix, and passes with the fix.
• Performed manual integration tests and verified the expected headers in the debug logs (results below)

Manual integration test results:

Without fix (response_checksum_validation=when_required)

GET
/tox.ini

amz-sdk-invocation-id:REDACTED
amz-sdk-request:attempt=1
content-length:0
host:REDACTED
if-match:"REDACTED"
range:bytes=0-33481
x-amz-checksum-mode:enabled
x-amz-content-sha256:UNSIGNED-PAYLOAD
x-amz-date:20260730T202516Z
x-amz-security-token:REDACTED

amz-sdk-invocation-id;amz-sdk-request;content-length;host;if-match;range;x-amz-checksum-mode;x-amz-content-sha256;x-amz-date;x-amz-security-token
UNSIGNED-PAYLOAD

Without fix (response_checksum_validation=when_supported)

GET
/tox.ini

amz-sdk-invocation-id:REDACTED
amz-sdk-request:attempt=1
content-length:0
host:REDACTED
if-match:"REDACTED"
range:bytes=0-33481
x-amz-checksum-mode:enabled
x-amz-content-sha256:UNSIGNED-PAYLOAD
x-amz-date:20260730T202857Z
x-amz-security-token:REDACTED

amz-sdk-invocation-id;amz-sdk-request;content-length;host;if-match;range;x-amz-checksum-mode;x-amz-content-sha256;x-amz-date;x-amz-security-token
UNSIGNED-PAYLOAD

With fix (response_checksum_validation=when_required)

GET
/tox.ini

amz-sdk-invocation-id:REDACTED
amz-sdk-request:attempt=1
content-length:0
host:REDACTED
range:bytes=0-8388607
x-amz-content-sha256:UNSIGNED-PAYLOAD
x-amz-date:20260730T202352Z
x-amz-security-token:REDACTED

amz-sdk-invocation-id;amz-sdk-request;content-length;host;range;x-amz-content-sha256;x-amz-date;x-amz-security-token
UNSIGNED-PAYLOAD

With fix (response_checksum_validation=when_supported)

GET
/tox.ini

amz-sdk-invocation-id:REDACTED
amz-sdk-request:attempt=1
content-length:0
host:REDACTED
if-match:"REDACTED"
range:bytes=0-33481
x-amz-checksum-mode:enabled
x-amz-content-sha256:UNSIGNED-PAYLOAD
x-amz-date:20260730T203001Z
x-amz-security-token:REDACTED

amz-sdk-invocation-id;amz-sdk-request;content-length;host;if-match;range;x-amz-checksum-mode;x-amz-content-sha256;x-amz-date;x-amz-security-token
UNSIGNED-PAYLOAD

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