fix: stop wiping unread counts for spaces and unchanged rooms in incremental recompute

[eleboucher]

Description

Fixes two regressions from #1253 that caused spaces and channels to lose unread badges after the incremental unread recompute landed.

1. getUnreadInfosForRooms deleted space rooms instead of skipping them. The RESET path (getUnreadInfos) skips spaces, but the new incremental path pushed them to the deleted list. Space unread is derived from children via parent propagation in the atom reducer, a direct DELETE wiped the aggregated count, and non-dirty children never re-PUT to restore it.
2. Empty [] account_data rooms were marked dirty. The server echoes every subscribed room in the account_data extension with an empty array when nothing changed. The old code added all of them to dirtyRoomIds, marking ~150 rooms dirty per sync cycle and defeating the incremental optimization.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).

[7w1]

Empty [] account_data rooms were marked dirty. The server echoes every subscribed room in the account_data extension with an empty array when nothing changed. The old code added all of them to dirtyRoomIds, marking ~150 rooms dirty per sync cycle and defeating the incremental optimization.

Not 100% sure but i think this might actually be a c10y bug? Doesn't happen on morg and I think i stumbled on it while implementing some other sliding sync things and its very odd. Server probably shouldn't be returning a list of unchanged empty rooms in the extension

[7w1]

also fix checks :(