fix(chat): join the room on resume, as the counterpart of leaving on pause

[ToteMeiSter]

Problem

After leaving a chat the app sometimes joins the room again 35-300 ms later (DELETE .../participants/active, then POST .../participants/active). The session stays in the room until the next leave, minutes later. While its last ping is fresh the server treats the user as online and sends no push for new messages.

The chat leaves the room in onPause, but joined it from the first capabilities state after a reset to the initial load. ChatViewModel.leaveRoom and every onSaveInstanceState reset getCapabilitiesViewState, so the next refresh of the conversation emitted GetCapabilitiesInitialLoadState and ChatActivity joined the room. LiveData delivers to a started observer, so a refresh between onPause and onStop joined the room right after it was left. onDestroy does not leave it. After a return from the background the join also depended on such a refresh.

Fix

• onResume calls ensureJoined() once the first initial load of the screen is done (the conversation can be set by the message list before the capabilities are loaded). The initial load observer calls it only while the screen is resumed, for the first open. ChatViewModel.claimRoomJoin() allows one join per resume and is released in ChatViewModel.onPause, where the pending join is disposed as well.
• leaveRoom no longer resets the capabilities state. onSaveInstanceState resets it only when isChangingConfigurations is true, where the new activity needs the initial load to build its UI; a stop without a configuration change keeps the state.
• The initial load no longer runs on every return, so onResume repeats what depends on the current state: shared files of a new intent, focus of the input, scheduled messages, upcoming event, out-of-office status and the room refresh timer. The deletion warnings of event and ended-call rooms are not shown again on a return.

Tests

ChatViewModelRoomJoinTest uses the real ChatViewModel with mocked repositories: a refresh of the conversation after leaveRoom is not an initial load, and leave, refresh, return without a configuration change joins exactly once per resume. It also covers a configuration change. The first two tests fail on master. The wiring in ChatActivity (onResume, the initial load flag, the isChangingConfigurations check in onSaveInstanceState) is not covered by a unit test.

Known remaining issues

• Back right after opening, while the join request is in flight: the request is disposed in onPause, no leave is sent, the session expires after about 30 s. Same as before.
• A failed join is not retried before the next resume.
• If the screen is left before the first capabilities load, nothing is joined or refreshed on the return until that load arrives.
• Split screen on API 26-28: the chat joins when it gets the focus, as it leaves when it loses it.

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not needed
• 🔖 Capability is checked or not needed
• 🔙 Backport requests are created or not needed: /backport to stable-xx.x
• 📅 Milestone is set
• 🌸 PR title is meaningful

🤖 AI

• The content of this PR was partly or fully generated using AI (Claude Code, Assisted-by trailer in the commit)

Tested on

• Huawei DEL-LX9: tested
• Samsung SM-S926B: tested

🤖 Generated with Claude Code