fix: keep one-per-line layout of short multiline __all__ under trailing-comma profiles

[Manny7717]

What

--sort-reexports rewrote a short multi-line __all__ (one element per line, trailing comma) into a single line whenever the sorted result happened to fit within line_length — even under --profile black, dropping the trailing comma and the one-per-line style that black itself preserves. Reported in #2578.

# before
__all__ = (
    "FirstClass",
    "SecondClass",
)
# after --sort-reexports --profile black
__all__ = ("FirstClass", "SecondClass")

Change

isort.literal now preserves the vertical block form when the original literal was multi-line AND the active config uses trailing commas (include_trailing_comma, set by the black/comma-style profiles). Single-line sources and default-config behavior are unchanged — the pre-existing tests encoding the collapse behavior for the default config still pass.

Tests

• test_sort_reexports_preserves_short_multiline_all_issue_2578: the issue's repro stays one-per-line and sorted, trailing comma intact. Fails on the previous implementation (verified).
• test_sort_reexports_keeps_short_single_line_all_issue_2578: single-line __all__ is sorted in place and stays single-line.
• All existing reexport tests (incl. the black-stability fixpoint test) pass; full suite 623 passed, 4 pre-existing env-dependent failures unrelated to this change.

[Manny7717]

Closing as a duplicate of #2605, which implements the same fix for the same issue (#2578) with the same mechanism — a preserve-trailing-comma flag threaded through the collection sorters, gated on include_trailing_comma. #2605 predates this PR (Aug 17 vs Aug 28) and is already under maintainer review, and I verified its head passes the full literal + reexport test suite locally. Deferring to it — thanks @PSR94 for the cleaner _has_trailing_comma formulation.