Fix patchedast region for implicitly-concatenated f-strings (fixes #879)

[stevemadere]

Description

Fixes an undocumented string parsing crash-inducing bug in _PatchingASTWalker._JoinedStr()

_JoinedStr builds its children as [start_quote, *FormattedValues, end_quote] and lets _handle consume each by forward search. The end-quote child therefore matches the first closing quote after the last interpolation -- which, in an implicit concatenation, closes only the part that held that interpolation, not the whole concatenation.

The node's region was then understated and, worse, the scanner was left parked inside source text that had not been recognised as a string literal. A # in a trailing part -- f"{a} one " f"#52 two" -- then reads to _good_token as the start of a real comment, so the following ) is skipped as commented-out and consume raises MismatchedTokenError (issue #879).

The truncation itself predates the #: a hash-free concatenation such as f"{a} one " f"52 two" also produced a short region, it just failed silently because nothing downstream needed to match a token past the truncation point.

consume_string already measures the full concatenation before _JoinedStr rewinds the scanner, so close the gap from its end offset rather than re-scanning. Plain (non-f) strings were never affected because _Str consumes via that same pattern.

Fixes #879

Checklist (delete if not relevant):

• I have added tests that prove my fix is effective or that my feature works
• I have updated CHANGELOG.md
• The documentation already implied it worked this way by neglecting to mention the crash. ;-)