[`pyupgrade`] Fix handling of `\N` in raw strings (`UP032`) (#22149)
**Summary:**
Fixes UP032 autofix incorrectly converting raw strings with `\N{...}` to
f-strings, which changes semantics and causes runtime errors.
Fixes #22060
## Test Plan
- Added test case for raw strings with \N{...}
- Regular strings with \N{...} still autofix correctly
- All 119 pyupgrade tests pass
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Co-authored-by: Brent Westbrook <brentrwestbrook@gmail.com>