fix(threads): show new thread immediately instead of only after reopening chat

[mahibi]

Starting a new thread showed the message as a normal chat message until the chat was reopened. Two gaps caused this:

• OfflineFirstChatRepository.sendChatMessage() discarded the server's response after a successful send and only updated the temp message's sendStatus, so the correct isThread/threadId/threadTitle from the server was never persisted until a later resync (e.g. on reopening the chat). Now the authoritative response is persisted right away, replacing the temp row.
• The optimistic temp message itself did not carry isThread/ threadTitle at all, and ChatViewModel.handleThreadMessages() misclassified that temp message (isThread=true, threadId=null, not yet confirmed by the server) as a thread reply and filtered it out of the normal chat view.

Assisted-by: Claude Code:claude-sonnet-5

🖼️ Screenshots

🏚️ Before	🏡 After
B	A

🚧 TODO

• ...

🏁 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 (if it should be in the changelog: is it meaningful to users?)

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI

[github-actions]

APK file: https://github.com/nextcloud/talk-android/actions/runs/33855916262/artifacts/9930387841
To test this change/fix you can simply download above APK file and install and test it in parallel to your existing Nextcloud app.
(please click on link to get QR code displayed)