fix: Don't attempt to serialize custom join rules

[poljar]

This is not supported by Ruma. The join_rule field, despite being defined as a pure string, can have associated data to it based on the join rule variant.

This means that custom and unknown enum variants might lose data when reserializing.

Let's just skip the serialization of custom join rules in the RoomInfo, the concrete value is still available in the state store, it's just not kept at hand in the RoomInfo.

This closes #5883 and #2949.

I have checked, the other enums that can encounter custom variants, i.e. HistoryVisibilty are not problematic as they have a working Serialize implementation for all variants, even the custom one.

• Public API changes documented in changelogs (optional)

[codspeed-hq]

CodSpeed Performance Report

Merging #5924 will not alter performance

_{Comparing poljar/skip-custom-join-rule-serialization (32d51a5) with main (2412403)}

Summary

✅ 50 untouched

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.65%. Comparing base (b4d7881) to head (32d51a5).
⚠️ Report is 20 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk-base/src/room/room_info.rs	66.66%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5924   +/-   ##
=======================================
  Coverage   88.65%   88.65%           
=======================================
  Files         363      363           
  Lines      104739   104739           
  Branches   104739   104739           
=======================================
+ Hits        92852    92861    +9     
+ Misses       7533     7522   -11     
- Partials     4354     4356    +2     

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[Hywan]

Fantastic, thanks for finding the true reason of this bug.