feat(room_list): make RoomListService updates subscriptions non-destructively

[Johennes]

With my Filament hat on.

Fixes: #6620

• I've documented the public API changes in the appropriate changelog files
  (see Writing changelog entries).
• This PR was made with the help of AI.

[codecov]

Codecov Report

❌ Patch coverage is 96.19048% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.88%. Comparing base (7b2b439) to head (f54d6c6).
⚠️ Report is 62 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/sliding_sync/mod.rs	96.15%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6652      +/-   ##
==========================================
+ Coverage   89.86%   89.88%   +0.01%     
==========================================
  Files         396      396              
  Lines      110260   110360     +100     
  Branches   110260   110360     +100     
==========================================
+ Hits        99088    99197     +109     
+ Misses       7395     7389       -6     
+ Partials     3777     3774       -3     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

---

_{Comparing Johennes:johannes/resubscribe-rooms (f54d6c6) with main (cc670dc)}

[Hywan]

Great! Almost there!

[Hywan]

I think we are good! You can rebase your fixup commits, and expand the changelog description a bit, and I'll be happy to merge i!t

[Johennes]

Sorry for the delay in getting this patched up. I have extended the changelog and rebased all the fixups away.

[Hywan]

LGTM, thanks!