Fix parsing of raw f-strings with backslash before brace

[worksbyfriday]

Summary

Fixes #207. Raw f-strings like rf"\{x}" fail to parse because the tokenizer treats \{ as a backslash escape sequence, consuming the { and preventing it from opening an f-string expression.

Root Cause

The f-string content regex (fstring_string_single_line) includes \\[^\r\nN] which matches a backslash followed by any non-newline character. This is correct for non-raw f-strings where \n, \t, etc. are escape sequences. But in raw f-strings, backslash is a literal character -- \{ should be tokenized as literal \ followed by expression-opening {.

Parso used the same regex for both raw and non-raw f-strings because FStringNode only tracked the quote character, not the prefix.

Fix

• Add is_raw flag to FStringNode
• Store the raw flag in fstring_pattern_map alongside the quote
• Add separate raw-aware regex patterns (fstring_string_single_line_raw, fstring_string_multi_line_raw) that treat backslash as an ordinary character
• Select the appropriate regex in _find_fstring_string based on tos.is_raw

Tests

Added 8 test cases for raw f-strings with all prefix case variations (rf, Rf, rF, fR, Fr) and both single and triple-quoted forms. All 1812 tests pass.

[davidhalter]

The linter does not pass. The coveralls error is obviously fine. Something is wrong with the service.

Since this is likely generated/assisted with AI, please make sure to think about this problem well enough so I don't have to. It really really wastes my time and makes me mad if people don't. Also I would personally recommend to not use AI for contributions, they are in my experience just sub-par and you don't learn anything yourself doing it that way.

[worksbyfriday]

Understood and respected. Thank you for explaining your position clearly. Closing this.