Fix: a duplicate group invite demoted a member who had already joined

[mpretty-cyro]

Issue #2215, reported with a reliable repro and against every Session Android version.

Receiving side

GroupManagerV2Impl.handleInvitation had no check for a group we are already in. It built a fresh ClosedGroupInfo and wrote it over the existing record, so a second invitation set invited back to true and reset joinedAtSecs to 0 — a joined member was demoted to "invited" and their join time was lost.

handlePromotion already guards this case with if (group == null), and the invitation path now does the same thing — but conditionally, not literally. A literal group == null check would break re-invitation after a kick: handleKicked keeps the record with kicked = true, and an invitation that rebuilds it is the only way back in. The guard is therefore "already joined" — present, and not invited, kicked or destroyed.

Admin side

inviteMembersInternal called setInvited() whatever state the member was in, so re-inviting someone who had already accepted reset them in the group members config too. It now skips that for INVITE_ACCEPTED and PROMOTION_ACCEPTED. Other promotion states are left exactly as they were.

Tests

GroupManagerV2ImplTest, synthetic group config only:

• a second invitation for a joined group writes nothing at all (verified at dangerouslyAccessMutableUserConfigs, so it covers both the membership flag and joinedAtSecs)
• a first invitation still writes, with invited = true — a control, so the negative test above cannot pass by never reaching the code

The first test was verified to fail with the guard reverted, reproducing the reported behaviour; the control passes either way, which is what makes the negative meaningful.

The admin half has no test: GroupMember's state setters are native, so a JVM unit test cannot reach them.

Unit suite: 300 pass, 0 fail (298 on dev plus these two).