Avoid moving back the lexer for triple-quoted fstring

[dhruvmanila]

Summary

This PR avoids moving back the lexer for a triple-quoted f-string during the re-lexing phase.

The reason this is a problem is that for a triple-quoted f-string the newlines are part of the f-string itself, specifically they'll be part of the FStringMiddle token. So, if we moved the lexer back, there would be a Newline token whose range would be in between an FStringMiddle token. This creates a panic in downstream usage.

fixes: #11937

Test Plan

Add test cases and validate the snapshots.

[github-actions]

ruff-ecosystem results

Linter (stable)

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Linter (preview)

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Formatter (stable)

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Formatter (preview)

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