fix: preserve trailing comments when sorting literals

[Manny7717]

Problem

Sorting a literal drops a trailing comment, and crashes when the comment (or a string inside the literal) contains =.

Repro (with --sort-reexports):

__all__ = ["b", "a"]  # public names

isort currently produces:

__all__ = ["a", "b"]

The comment # public names is silently deleted. With a comment containing = (or any literal containing = in a string), isort crashes instead:

__all__ = ["b", "a"]  # note: key = value

ValueError: too many values to unpack (expected 2, got 3)

This affects both --sort-reexports and the # isort: list / # isort: dict / # isort: tuple / # isort: set inline directives. Closes #2646.

Root cause

Two split("=") calls without a maxsplit:

1. isort/core.py reexport detection: _, rhs = stripped_line.split("=") — the comment # note: key = value contains two =, so the unpacking fails.
2. isort/literal.py assignment(): variable_name, literal = code.split("=") — same crash when the comment or a string element (e.g. ["a=b", "c"]) contains =.

Separately, assignment() rebuilt the line as <name> = <sorted literal> and then only re-appended the trailing whitespace of the original line (code[len(code.rstrip()):]), so anything that followed the literal — i.e. a trailing comment — was discarded.

Fix

• isort/core.py: split on the first = only (split("=", 1)).
• isort/literal.py:
  • split on the first = only;
  • parse the literal once with ast.parse(..., mode="eval") (still evaluated via ast.literal_eval), then use the value node's end position to re-append everything that followed the literal verbatim — trailing comments and any trailing whitespace survive the rewrite.

Tests

• tests/unit/test_literal.py (4 new): trailing comment preserved; trailing comment containing = preserved; literal with = inside a string sorts; trailing comment on a multi-line literal preserved.
• tests/unit/test_isort.py (3 new): end-to-end --sort-reexports and # isort: list cases, including a comment containing = (the exact crash from the issue).

Regression-proven: all 7 new tests fail on pre-fix main (4 crash with ValueError: too many values to unpack, 3 silently drop the comment) and pass with the fix. The no-comment path is unchanged (existing test_reexport_* suite still green).

Verification

• Full unit suite: 628 passed / 4 failed / 1 skipped — the 4 failures (issue_909, issue_938, issue_970, issue_1732) are pre-existing FileNotFoundError env noise, byte-identical on clean main.
• tests/integration/: 12 passed, 1 skipped.
• mypy isort/literal.py isort/core.py: clean.
• ruff check / ruff format --check / flake8 / isort self-check on changed files: clean.

Note: this PR intentionally does not change the separate "statement after the literal" handling (that's #2286 / PR #2627's scope); it only fixes the comment-drop and the = crash.

AI assistance disclosure: this contribution was developed with assistance from an AI coding agent (Hermes/Lappy on behalf of Manny7717). All changes were locally reproduced, regression-tested (failing before the fix), and verified against the full test suite before opening.