fix(useGetMessages): always start message polling

[Antreesy]

☑️ Resolves

• Might help with While being in a call, I dont receive messages sent during the call #15868

One of the cases to reproduce (may be something else):

• add sleep(5) to ChatController#getMessageContext
• join the room
• post a message before contextRequest was finished loading
• contextId shifted to a new message
• contextRequest 1 is cancelled, polling is never started
• contextRequest2 is started, no more new messages

With this PR:

• use the same fallback to last read message + long polling
• if context request was cancelled, we at least have:
• lastMessage to show in the chat
• lastReadMessageId to start polling from it

🖌️ UI Checklist

🖼️ Screenshots / Screencasts

🏁 Checklist

• 🌏 Tested with different browsers / clients:
  • Chromium (Chrome / Edge / Opera / Brave)
  • Firefox
  • Safari
  • Talk Desktop
  • Integrations with Files sidebar and other apps
  • Not risky to browser differences / client
• 🖌️ Design was reviewed, approved or inspired by the design team
• ⛑️ Tests are included or not possible
• 📗 User documentation in https://github.com/nextcloud/documentation/tree/master/user_manual/talk has been updated or is not required

[Antreesy]

/backport to stable32