Only write a note for an RSVP that says something new

[KrisBraun]

When an attendee responds to a calendar invitation, the Gmail connector folds that
response onto the event's thread as a note. A plain acceptance carries nothing the
event's guest list does not already show — but attaching any note surfaces the
thread as unread for every recipient except the note's author. Accepting an
invitation therefore made the organiser's own event thread unread.

The unread field could not prevent this. Omitting it does not mean "leave read
state alone" — the thread has already been surfaced by the time that field is
read — so the connector's acceptance guard, and its initial-backfill guard, both
had no effect in practice.

What changes

A response becomes a note only when it says something new:

Response	Note
Declined / tentative	yes
Accepted, with a personal comment	yes
Accepted, reversing an earlier decline or tentative	yes
Accepted, nothing else	no

A plain acceptance produces no note, and its notification email is dropped rather
than becoming a thread of its own — so a conversation consisting only of such
acceptances no longer creates a thread at all. The event's guest list continues to
show who accepted.

Every emitted note now carries an explicit unread value, on both the live sync
and the deferred-retry path that handles responses arriving before their event has
synced. During the initial backfill that value is false, so folding a history of
past responses no longer resurfaces old events.

Reversals are tracked per attendee per occurrence: declining one occurrence of
a recurring meeting does not change how a later acceptance on a different
occurrence is treated.

Notes

• During the initial backfill the explicit read state applies to the connection
  owner. On a thread with several participants, other recipients may still be
  surfaced by later processing.
• composeRsvpNote is unchanged — note wording is the same as before.