Update update_senders when an update proposal is removed from the bundle

[stefunctional]

Description of changes:

update_senders is now updated when the list of update proposals changes to ensure they stay in sync.

Call-outs:

N/A

Testing:

A test was added to check that the resulting group state is correct when an invalid update proposal was submitted.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT license.

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (f74a5cc) 91.63% compared to head (1267e9c) 91.97%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #31      +/-   ##
==========================================
+ Coverage   91.63%   91.97%   +0.33%     
==========================================
  Files         167      167              
  Lines       30048    30244     +196     
==========================================
+ Hits        27535    27816     +281     
+ Misses       2513     2428      -85     

Files	Coverage Δ
mls-rs/src/group/mod.rs	95.43% <100.00%> (+0.26%)	⬆️
mls-rs/src/group/proposal_filter/filtering.rs	95.33% <80.00%> (-0.24%)	⬇️
mls-rs/src/tree_kem/mod.rs	96.33% <0.00%> (-0.10%)	⬇️

... and 9 files with indirect coverage changes

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[mulmarta]

Thanks for finding this!