Sort extra keys when formatting ForbiddenExtraKeysError

[MaxFreedomPollard]

transform_error reports the extra keys of a ForbiddenExtraKeysError in a different order on every run. ForbiddenExtraKeysError.extra_fields is a set, and format_exception in src/cattrs/v.py line 44 joins it as it comes: res = f"extra fields found ({', '.join(exc.extra_fields)})". String hashing is randomized per process, so the message text changes between runs as soon as there is more than one extra key, and anything that compares or snapshots the output of transform_error fails intermittently.

ForbiddenExtraKeysError.__str__ in src/cattrs/errors.py line 141 already sorts the same set, so the two renderings of a single error disagree with each other too.

from attrs import define
from cattrs import Converter, transform_error

@define
class C:
    a: int

c = Converter(forbid_extra_keys=True, detailed_validation=True)
try:
    c.structure({"a": 1, "b": 2, "c": 3, "d": 4, "e": 5}, C)
except Exception as exc:
    print(transform_error(exc))
    print(str(exc.exceptions[0]))

Three runs of that, on main:

['extra fields found (c, d, b, e) @ $']
Extra fields in constructor for C: b, c, d, e
['extra fields found (e, c, d, b) @ $']
Extra fields in constructor for C: b, c, d, e
['extra fields found (d, c, b, e) @ $']
Extra fields in constructor for C: b, c, d, e

The fix is to sort in format_exception as well. The regression test uses five extra keys, because the existing test_class_errors only ever produces one and so cannot see an ordering at all.

Verification, all on the branch unless stated otherwise:

uv run --all-extras --group test --group lint pytest tests/test_v.py::test_extra_keys_are_sorted -q under PYTHONHASHSEED 1 through 8 with the test applied to unmodified main: 8 failures out of 8, for example At index 0 diff: 'extra fields found (c, d, b, e) @ $' != 'extra fields found (b, c, d, e) @ $'. The same command on tests/test_v.py with the fix under PYTHONHASHSEED 1 through 12: 12 passes out of 12.

uv run --all-extras --group test --group lint pytest -n 2 tests -q on 3.11: 941 passed, 15 xfailed.

uv run -p <version> ... pytest tests/test_v.py -q on 3.10, 3.11, 3.14 and pypy3.10: 10 passed each. I ran pypy without --all-extras because ujson has no pypy wheel on macOS, which is unrelated to this change.

uv run -p python3.14 --group lint ruff check src/ tests bench: All checks passed. uv run -p python3.14 --group lint ruff format --check src tests docs/conf.py: 115 files already formatted.

[codspeed-hq]

Merging this PR will not alter performance

✅ 64 untouched benchmarks

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_{Comparing MaxFreedomPollard:sort-forbidden-extra-keys (f0c5a80) with main (df4e4bb)}

[Tinche]

LGTM, thanks!