feat[ffi]: expose join_rules in OtherState::RoomJoinRules

[JoFrost]

Hello! As a follow-up to #5830, this PR exposes the room join rules in the OtherState::RoomJoinRules event for the FFI timeline.

It reuses the existing JoinRules type from the client module and converts the event content accordingly. This allows clients to inspect the room’s current join rule directly from the event. Like m.federate, this field was previously unavailable in the FFI variant of the SDK.

• Public API changes documented in changelogs (optional)

Signed-off-by:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.59%. Comparing base (1db4a4c) to head (af13f11).
⚠️ Report is 28 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5863   +/-   ##
=======================================
  Coverage   88.59%   88.59%           
=======================================
  Files         362      362           
  Lines      102323   102323           
  Branches   102323   102323           
=======================================
+ Hits        90653    90655    +2     
+ Misses       7448     7445    -3     
- Partials     4222     4223    +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[codspeed-hq]

CodSpeed Performance Report

Merging #5863 will not alter performance

_{Comparing JoFrost:main (af13f11) with main (f702364)}

Summary

✅ 50 untouched

[stefanceriu]

Looks good to me, thank you!