Don't fold source after an f-string with an unmatched literal bracket

[gadievron]

Description

simplify.real_code() folds all source after an f-string that contains an
unmatched literal bracket (e.g. f"[{x}", f"({n} items", an ANSI escape
f"\x1b[K{x}", an escaped brace f"{{", or a format spec like f"{x:>[}").

real_code deliberately leaves f-strings unblanked so their inner-expression
offsets stay valid. The subsequent implicit-continuation pass _parens then
counts the f-string's literal brackets too, so parens goes positive and
never returns to zero, and every newline to EOF is blanked. Everything below the
f-string collapses onto one logical line, so any offset-based analysis of that
code is wrong.

real_code's only consumer is worder, so the user-visible effect lands on
worder's header predicate: for a class-attribute def whose def follows
the f-string, find_definition resolves to the wrong location (or the call site)
and Rename silently renames the usages while leaving the def untouched (or
vice-versa) — no exception, no diagnostic. Module-level and nested defs are
immune (they don't go through that predicate).

>>> from rope.base import simplify
>>> simplify.real_code('f"[{x}"\ndef g():\n    return 1\n')
'f"[{x}" def g():     return 1 '   # 3 newlines collapsed to spaces

Fix

_blank_fstring_literal_brackets() tokenizes the source (PEP 701, Python 3.12+)
and blanks every ()[]{} character that is not covered by an OP token —
i.e. the literal/format-spec brackets, and bracket characters that are literal
data inside a nested string. Real expression brackets (f"{[1,2][0]}",
f"{x:{width}}") are OP tokens and are left untouched. Blanking is
length-preserving (one char → one space), so all offsets are unchanged; it is
required so the literal bracket stops swaying _parens. On Python < 3.12 an
f-string is one opaque STRING token and can't be split, so the pass is a strict
no-op; a TokenError/SyntaxError fallback leaves the source unchanged on
incomplete/invalid mid-edit buffers.

Tests are version-gated with only_for_versions_higher("3.12") and cover each
trigger class above plus an end-to-end find_definition + Rename regression, a
set of preservation controls (real brackets untouched), a module-level immunity
control, and the invalid-input fallback.

Known limitation

The blanking pass is gated on rope's existing regex-based f-string detection
(ignored_regions), which recognises single-line and triple-quoted f-strings but
not single-quoted multiline f-strings (f"[{1 +⏎2}"); those still fold as
before. Dropping the gate (tokenizing unconditionally) would close the remaining
case — happy to do that here instead if you'd prefer.

Related

Sibling of the still-open #499 (patched_ast sets region[0]=None on an
unmatched bracket in an f-string): same trigger family, but a different code path
(patched_ast does not call real_code). This PR does not fix #499. Same
desync class as the false-triple-quote bug #248 (fixed by #309).

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Checklist

• I have added tests that prove my fix is effective
• I have updated CHANGELOG.md