Verify membership when test if name is ambiguous

[multisme]

Verify if the membership is not Leave when checking if a name in a room is ambiguous.

Signed-off-by:
multi

Fix #5774

[multisme]

It's pretty straightforward but should I had some tests?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.42%. Comparing base (588d604) to head (47488fd).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5780      +/-   ##
==========================================
- Coverage   88.43%   88.42%   -0.01%     
==========================================
  Files         360      360              
  Lines       99796    99797       +1     
  Branches    99796    99797       +1     
==========================================
- Hits        88250    88246       -4     
- Misses       7405     7411       +6     
+ Partials     4141     4140       -1     

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[codspeed-hq]

CodSpeed Performance Report

Merging #5780 will not alter performance

_{Comparing multisme:fix_check_if_user_really_ambiguous (47488fd) with main (7185fcb)}

Summary

✅ 50 untouched