Fix # fmt: skip ignored in deeply nested expressions

[Nikhil172913832]

Description

Closes #4730

Black was ignoring # fmt: skip in deeply nested expressions (e.g., multi-part if-clauses). This change updates the tree climbing logic to correctly find the start of the line and all relevant siblings, ensuring that # fmt: skip works as intended even in complex nested cases.

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• Add new / update outdated documentation?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 (815b840) to main (1b342ef).

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What is this? | Workflow run | diff-shades documentation

[cobaltt7]

The diff-shades doesn't look right, it looks like some changes from multiline_string_handling have been reverted. Could you take a look at that? (click the "available in the logs") link

Also, as a heads up, we're going to be releasing 25.12.0 over the weekend, and we want to keep 26.1.0 limited to purely stablizing preview styles as much as possible, so this probably won't get merged until after those are released.

[Nikhil172913832]

Thanks for the review. I’ve made the required changes. Happy to wait for the right window after the upcoming releases.

[cobaltt7]

Since this looks good and we haven't released yet, I'll go ahead and merge this now. Thanks!