Fix string_processing error with unassigned long string literal with trailing comma

[Ankitsinghsisodya]

Description

This PR fixes issue #4912 where string_processing would crash with an INTERNAL ERROR
when processing an unassigned long string literal with a trailing comma (creating a
one-item tuple).

The Problem: When a line contains only a long string literal followed by a trailing
comma (making it a one-item tuple) and is not inside brackets, StringSplitter would
split the string into multiple implicitly concatenated strings. This created multiple
expressions where only the last has the comma, breaking AST equivalence:

# Input
"A long string that exceeds line length...",

# Incorrect output (before fix)
'A long string that exceeds line length'
'continuation of the string...',  # Only this line has the comma!

The Fix:

1. StringSplitter.do_splitter_match() now detects the STRING + COMMA pattern (bare
   one-item tuple not inside brackets) and returns an error, deferring to
   StringParenWrapper
2. StringParenWrapper._trailing_comma_tuple_match() is a new method that matches this
   specific pattern
3. The string is wrapped in parentheses before splitting, preserving the tuple
   semantics:

# Correct output (after fix)
(
    "A long string that exceeds line length"
    " continuation of the string..."
),

Checklist - did you ...

• Implement any code style changes under the --preview style, following the
  stability policy? > Yes, string_processing is an unstable feature enabled via
  --preview --enable-unstable-feature=string_processing

• Add an entry in CHANGES.md if necessary? > Added under "Preview style" in the
  Unreleased section

• Add / update tests if necessary? > Added test case in
  tests/data/cases/preview_long_strings.py

• Add new / update outdated documentation? > No documentation changes needed - this
  is a bugfix for existing functionality

Fixes #4912
closes #4917

[github-actions]

diff-shades reports zero changes comparing this PR (2fcfba1) to main (c3cc5a9).

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[cobaltt7]

Everything looks good here, thanks! Will merge after the 2026 style is stabilized.