Support the GroupsV2 "end group" (terminate) feature

[FailSpy]

Signal recently added the ability for a group admin to permanently terminate a group ("End group" in the app UI): once it's terminated the group is read-only for everyone -- nobody, admins included, can send messages or start calls -- and members keep the existing history. Other clients silently drop any message sent to a terminated group. The pinned signal-network already carries the protobuf for it (DecryptedGroup.terminated, plus the TerminateGroupAction change), so signal-cli just wasn't reading or offering it yet. This wires it up both directions.

Seeing that a group is terminated

Today signal-cli can't tell a terminated group from a live one, and it'll report a sent message as SUCCESS even though every other client drops it. This reads DecryptedGroup.terminated through GroupInfo.isTerminated() and:

• surfaces it in listGroups (JSON, and both the default and --detailed plaintext lines) and as the DBus IsTerminated property
• refuses to send to a terminated group locally -- messages, reactions, typing and group stories -- rather than send one other clients ignore
• drops incoming messages addressed to a terminated group (in isNotAllowedToSendToGroup, alongside the announcement-group handling)

The send refusal mirrors the existing isAnnouncementGroup wiring, with one difference: it's unconditional. Announcement-only exempts admins; a terminated group blocks everyone.

On the listGroups output: I put Terminated on the default (non-detailed) line, right after Blocked. Blocked is almost always false yet already lives there, so that line is a status-at-a-glance row, not an only-show-non-defaults one. Terminated is a peer of Blocked -- both mean "this isn't a usable conversation" -- and since a terminated group otherwise looks completely normal, the glanceable line is where it's most worth surfacing. Happy to move it behind --detailed if you'd rather leave the default line untouched.

Terminating a group

Adds a terminateGroup command (admin-only, v2 groups) that sends the TerminateGroupAction via the library's own createTerminateGroup() and notifies the members. It follows the quitGroup shape end to end -- GroupHelper.terminateGroup -> Manager/ManagerImpl -> the terminateGroup command (command line + JSON-RPC) and the DBus Group.terminateGroup method -- with the same conflict-retry as updateGroup.

Tests / scope

Added a unit test for the terminated model reads and the Group record wiring, in the existing JUnit style (no new deps). The send-path guards aren't unit-covered -- there's no existing test path for the outgoing/send layer to build on -- but the isTerminated() signal they rely on is.

[AsamK]

Thanks, looks good.
Just the naming is inconsistent, please use terminate instead of end everywhere