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Prevent string merger from creating unsplittable long lines (#5096)

* Prevent string merger from creating unsplittable long lines (#4510)

When multi-line strings are followed by a pragma comment like
`# type: ignore` on a surrounding bracket, the StringMerger would merge
them into a single long line that could not be split back due to the
pragma comment.  For example:

    (
        "A long string "
        "continued here."
    )  # type: ignore

Would become one enormous line that violates the line length limit.

The root cause is that `_validate_msg()` only checked for pragma comments
on the string leaves themselves, not on surrounding tokens like closing
brackets. The fix extends the validation to also check for pragma
comments on tokens following the string group (particularly closing
brackets), refusing to merge when such comments are present.

Fixes #4510.

* Add CHANGES.md entry for string merger pragma comment fix

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

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