fix: preserve time and offset in ISO datetime strings

[ryanduguid]

validate_io_timestamp() parses ISO datetime strings, then combines the result with midnight. This discards the supplied time, fractional seconds and UTC offset. For example, with TIME_ZONE='America/New_York', committing 2024-07-01T00:35:27.123456+10:00 saves 2024-07-01T04:00:00Z instead of 2024-06-30T14:35:27.123456Z.

Return parsed datetimes before the date-only conversion. Naive datetime strings use the current timezone when USE_TZ=True and remain naive otherwise. Date-only strings retain their existing midnight behaviour. Regression coverage checks UTC and positive/negative offsets, fractional seconds, timezone settings, date-only inputs and the timestamp saved by commit_txs().

Validation on Windows with Python 3.12.10 and Django 6.0.1, using uv sync --locked --python 3.12:

• The new regression cases fail on the original implementation and pass with this fix.
• python manage.py test django_ledger.tests.test_io --noinput: 13 tests pass.
• python manage.py test --noinput: 60 tests, 14 failing or erroring. Untouched upstream 5b4787b runs 55 tests with the same 14 failing test names (OFX helpers, transaction forms, UI messages and query counts). test_bill_detail depends on generated data: on both revisions it fails either its query count or a later account-detail URL reverse, so the failure/error split varies between runs.
• Scoped Pylint with the repository's error-only options reports the same 10 existing mixin no-member errors in the changed module and upstream source. The test file has no errors.
• git diff --check passes.

The complete Linux/Python 3.9 CI and full lint workflow were not reproduced locally. The manifest requires Python 3.11 or newer, and the lint workflow references an absent brickbox/ directory. This change affects newly parsed timestamps; it does not repair previously saved entries.