Invite people to an event, and give them somewhere to meet

[rutgerhofste]

calendar_create_event could make an appointment but not a meeting: no way to
invite anyone, and no way to ask for a conference link. This adds both to the
seam, and lets a backend say it cannot do them.

What changed

• attendees on calendar_create_event — email addresses; the backend
  invites them. Takes the shapes mail recipients take (list, bare address,
  comma-separated) through the same as_str_list, deduped case-insensitively.
• online_meeting — asks the backend to mint a conference link. It comes
  back as join_url, on both the create result and calendar_read_event.
• supports_attendees / supports_online_meeting on CalendarProvider,
  read as getattr(..., False) — the same shape as the mail pillar's
  supports_outgoing_attachments, and for the same reason.
• CalDAV answers False to both; Microsoft Graph (admin package) will answer
  True and is the first backend that can do either.

Why the capability flags rather than a best-effort write

Writing an ATTENDEE line into an iCalendar object does not invite anybody.
Delivering the invitation is server-side scheduling (RFC 6638) — some CalDAV
servers implement it, others quietly do not, and nothing in the protocol lets a
client tell which one it is talking to. A caller who believes they invited five
people and invited none is worse off than one whose tool call refused, so the
refusal names the alternative in the same sentence. This is the existing
attachments doctrine applied to the calendar pillar.

An online meeting has no CalDAV equivalent at all: a plain calendar server
stores conferencing details and creates none.

The read-back

The backend mints the link while creating the event, so the only way to it is a
read. calendar_create_event does that read when online_meeting=true, and
swallows a failure with a logged warning: the event exists by that point, and
reporting the tool call as failed invites a retry that books it twice.

Deliberately not here

calendar_update_event still cannot add attendees or turn an existing event
into a meeting. Graph refuses the latter outright (isOnlineMeeting cannot be
changed once set), and the former is a separate decision about whether an update
re-sends invitations. Worth doing; not worth bundling.

Tests

tests/test_calendar_invitations.py — 13 cases, mostly the refusals: attendees
and online meetings against a backend that cannot, a provider that predates both
properties (getattr default), the plain-event path staying untouched, plus the
capable path, the normalisation, the join link, and a read-back that fails
without failing the creation.

make test 168 passed · make lint clean.

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