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Fix crash when f-string follows # fmt: off inside brackets (#5097)

* Fix crash when f-string follows # fmt: off inside brackets (#4511)

When an f-string appeared after a `# fmt: off` comment inside brackets,
Black would crash with an AssertionError about invalid string prefix
characters.

The root cause: `normalize_fmt_off` (which runs before line generation)
converts content between `# fmt: off` and `# fmt: on` into
STANDALONE_COMMENT nodes. When this content is inside an f-string node,
the f-string's children get replaced with a STANDALONE_COMMENT leaf.
However, `visit_fstring` still tried to call `fstring_tstring_to_string`
on the modified node, producing a "string" value like
`# fmt: off\nf"""\n"""` which fails the string prefix assertion.

The fix adds a guard in `visit_fstring` (and `visit_tstring`) to detect
when the node has been modified by `normalize_fmt_off` and fall back to
`visit_default` instead of attempting the f-string-to-string conversion.

Fixes #4511.

* Add unindented test cases for f-string with # fmt: off inside brackets

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

* Add changelog entry for f-string fmt: off crash 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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