feat(messages): expose sender name and send time on Message

[kasperhartwich]

The API returns sender.fullName and sendDateTime for every message, but
Message dropped both, so neither the library nor the JSON output of
aula messages could say who wrote a message or when it was sent.

Changes:

• Message gains sender_name: str | None (from sender.fullName) and
  send_datetime: datetime | None (from sendDateTime, parsed with
  parse_api_datetime, like Post's timestamps). Both default to None for
  a missing, empty or unreadable value.
• Both new fields are keyword-only, so the positional order is unchanged:
  Message(id, content_html, attachments, _raw) means what it did before.
• dict(message) and --output json include them automatically, since
  AulaDataClass.__iter__ covers every field except _raw.

Tests:

• tests/models/test_message.py: sender and time read from API data;
  missing sender; sender as None, {}, {"fullName": None},
  {"fullName": ""} and a non-dict; unreadable timestamps (None, "",
  garbage, a number); positional construction unchanged.
• Full suite: 825 passed, 4 skipped. ruff format --check and ruff check
  clean.
• Checked against a live account: messages now show their sender and send
  time in aula --output json messages.