feat(ffi): expose required_state configuration on SyncServiceBuilder

[lucasfeijo]

Add SyncServiceBuilder::with_required_state() so FFI consumers can request additional state events during sliding sync beyond the hardcoded defaults.

Motivation

The RoomListService hardcodes DEFAULT_REQUIRED_STATE and DEFAULT_ROOM_SUBSCRIPTION_EXTRA_REQUIRED_STATE, which cover common Matrix state events. However, applications that use custom state events have no way to include them in the sliding sync request through the FFI layer.

Tested by integrating a build into our iOS application with custom state event types.

• I've documented the public API Changes in the appropriate CHANGELOG.md files.
• This PR was made with the help of AI.

Signed-off-by:

[codecov]

Codecov Report

❌ Patch coverage is 84.00000% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.83%. Comparing base (3d29db5) to head (7c4f2b8).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk-ui/src/sync_service.rs	69.23%	7 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6431   +/-   ##
=======================================
  Coverage   89.83%   89.83%           
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  Files         378      378           
  Lines      104214   104256   +42     
  Branches   104214   104256   +42     
=======================================
+ Hits        93616    93658   +42     
- Misses       7008     7010    +2     
+ Partials     3590     3588    -2     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

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_{Comparing lucasfeijo:feat/ffi-sync-service-required-state (7c4f2b8) with main (3d29db5)}

[bnjbvr]

The feature makes sense to me, but at this point it's badly implemented (by the AI):

• the introduced test doesn't really test the new feature, as it doesn't check the request received by the server. Look at other tests which do that, and make sure to assert on the extra requested states in the mock server's received requests.
• it isn't clear if the extra required states applies to all rooms or only rooms that one is subscribed to. Make it clearer in the function name, and make sure to add a test that the extra requested state events aren't requested in the other mode.
• it isn't clear if a state event type that was already present in the required list will be duplicated, or overridden (and which one will be overridden; the user-provided one, or the default). I think one reasonable behavior would be that the user-provided ones should override the predefined ones. Please add a test asserting this.