Mark message prefetch requests and let the server turn preloading off

[AndyScherzinger]

Stack — review and merge bottom-up. Each PR targets the branch below it, so it shows its parents' commits until they merge.

1. #6744 — modifiedSince delta sync
2. #6745 — periodic background refresh
3. #6746 — foreground refresh loop
4. #6763 — prefetch marking + mobile-preload-chat capability ← you are here

The client fetches a conversation's messages before the user asks for them — after a push notification, and after a conversation list sync. On the wire those requests look exactly like the ones a user staring at a chat screen is waiting for, and the server has no way to say it would rather the client didn't make them at all.

Two commits, each answering one half of that.

prefetch=1 on the requests the catch-up makes

A query parameter rather than a header, so it lands in a standard access log without any server configuration. Verified against a live server: the response is byte for byte the one sent without it, so the parameter is inert and purely descriptive.

The flag travels the same route as markNotificationsAsRead, which already separates these two cases, and defaults to false. The helpers that build the request are shared with the chat the user has open (OfflineFirstChatRepository calls buildFieldMap too), so a flag set any higher up would have labelled the messages someone is actively reading as prefetched — inverting the distinction this exists to draw. There is a test for exactly that.

The mobile-preload-chat capability

Implements the client side of spreed#19509. Preloading is skipped only when the server reports the setting as false. A server that reports true, one that omits it, and one too old to know it at all all preload — an operator cannot have declined something they were never offered.

Checked at both entry points, mirroring how chat-keep-notifications is already handled: the conversation list skips the whole catch-up rather than asking room by room, and the syncer checks it too, because push notifications reach it without passing the list.

Scope

Only message preloading. The conversation list sync is not gated: the capability is about chat messages, and an operator turning message preloading off has not asked for unread counts to go stale.

🚧 TODO

• Verify the false path against a server that reports the capability — the test instance predates spreed#19509, so only the absent case could be exercised live

🏁 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