[pyupgrade] Fix UP030 to avoid modifying double curly braces in format strings

[danparizher]

Summary

Fixes #19348

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[ntBre]

Thanks! I think we can just extend the current regex and also handle an additional edge case at the same time.

[ntBre]

At the risk of going too wild with regular expressions, I think we can just handle this directly in FORMAT_SPECIFIER:

static FORMAT_SPECIFIER: LazyLock<Regex> = LazyLock::new(|| {
    Regex::new(
        r"(?x)
            (?P<prefix>
                ^|[^{]|(?:\{{2})+  # preceded by nothing, a non-brace, or an even number of braces
            )
            \{                     # opening curly brace
                (?P<int>\d+)       # followed by any integer
                (?P<fmt>.*?)       # followed by any text
            }                      # followed by a closing brace
        ",
    )
    .unwrap()
});

While I was here, I went ahead and set verbose mode on the regex (initial (?x)) and moved the comments on FORMAT_SPECIFIER into the regex itself.

With this change, we also have to pass along the $prefix in the calls below. For example:

                        string.value = arena.alloc(
                            FORMAT_SPECIFIER
                                .replace_all(string.value, "$prefix{$fmt}")
                                .to_string(),
                        );

I almost neglected the "even number of braces" check, but cases like this are valid:

"{{{0}}}".format(123)

I think this is verging on needing us to parse the whole format string manually, but if we can't come up with any other edge cases, I think the regex is fine. I'd recommend adding this as a fixable test case too.