Fix nested configure set subattribute updates

[fallintoplace]

Summary

Fix nested aws configure set updates so subattribute writes stop cleanly at section and indentation boundaries instead of crashing or rewriting same-indent sibling keys.

Fixes #10348.

Root cause

ConfigFileWriter._update_subattributes() read current_indent even when the current line did not match OPTION_REGEX, which caused an UnboundLocalError for empty nested blocks, comments, and immediate section headers.

The helper also evaluated nested-key updates before confirming that an option line was still inside the nested block, which let same-indent sibling keys get rewritten incorrectly.

What changed

• treat section headers as an immediate nested-block boundary
• skip non-option lines safely while scanning nested blocks
• stop at same-indent sibling options before attempting nested-key updates
• add regression tests for empty nested blocks, comment lines, immediate section headers, and same-indent sibling keys

Validation

• PYTHONPATH=/Users/hoangvu/Code/OSS/aws-cli /tmp/awscli-verify/bin/python -m pytest -q tests/unit/customizations/configure/test_writer.py