Improve fmt:skip handling in nested expressions with checks

[Nikhil172913832]

Description

Closes #4730

This PR addresses issue #4730, where # fmt: skip was being ignored or causing crashes in deeply nested expressions. The root cause is that Black's handling of fmt: skip converts AST nodes to string literals, which bypasses bracket tracking and can result in missing attributes like bracket_depth. This led to AttributeError exceptions, especially with extreme settings like --line-length=1.

Key changes:

• Added hasattr(leaf, "bracket_depth") checks in src/black/lines.py to prevent crashes when processing leaves generated by fmt: skip.
• Added a test (tests/data/cases/fmtskip_nested_expression.py) using the original code from the issue, without relying on the --no-preview-line-length-1 flag, as requested by maintainers.
• Updated CHANGES.md to document the fix.

Checklist - did you ...

• Implement any code style changes under the --preview style, following the
  stability policy?
• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?

[github-actions]

diff-shades reports zero changes comparing this PR (621b9ea) to main (2fd75b0).

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[Nikhil172913832]

Hi @cobaltt7, can you have a look at this PR when you get time.