fix: Parent hash validation could prematurely return success

[mulmarta]

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

❌ Patch coverage is 94.44444% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.50%. Comparing base (efc2611) to head (134e509).

Files with missing lines	Patch %	Lines
mls-rs/src/tree_kem/parent_hash.rs	94.44%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #315   +/-   ##
=======================================
  Coverage   88.49%   88.50%           
=======================================
  Files         182      182           
  Lines       28969    29002   +33     
=======================================
+ Hits        25636    25668   +32     
- Misses       3333     3334    +1     

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