[KushagraIndurkhya]

Issue #: #10743

Description of changes

In DoneResultSubscriber._is_existing_object_skipped, hasattr(exception, 'response') was checked and exception.response.get('Error', {}) was accessed directly. For exceptions like ReadTimeoutError, hasattr is True but exception.response is None. This resulted in an unhandled AttributeError: 'NoneType' object has no attribute 'get' inside _on_failure before FailureResult could be queued. Consequently, files_failed remained 0 and aws s3 cp exited with 0 (success) despite the upload failure.

This change:

1. Safely checks that exception.response and future.meta.call_args.extra_args are dictionaries before querying them.
2. Adds unit tests in tests/unit/customizations/s3/test_results.py covering ReadTimeoutError and malformed error structures.
3. Adds a functional test in tests/functional/s3/test_cp_command.py verifying that aws s3 cp returns exit code 1 when an upload fails with ReadTimeoutError.
4. Adds a changelog entry at .changes/next-release/s3-cp-exit-code.json.

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