Add ButtJointSequence exposure

[greschd]

Add the ButtJointSequence class, and a PrimaryPly class for the edge properties
of the primary_plies attribute.

Other changes:

• Add an allowed_types_getter parameter to define_polymorphic_linked_object_list
  with the same purpose as the existing allowed_types, except the types are evaluated
  only inside the getter.
  This was needed to avoid a circular import with the ModelingGroup, since
  define_polymorphic_linked_object_list is called at the module top-level.

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 8 lines in your changes missing coverage. Please review.

Project coverage is 93.29%. Comparing base (d3986e6) to head (774123c).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...nsys/acp/core/_tree_objects/butt_joint_sequence.py	91.86%	7 Missing ⚠️
.../_tree_objects/_grpc_helpers/linked_object_list.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #606      +/-   ##
==========================================
- Coverage   93.33%   93.29%   -0.04%     
==========================================
  Files          86       87       +1     
  Lines        4604     4698      +94     
==========================================
+ Hits         4297     4383      +86     
- Misses        307      315       +8     

Flag	Coverage Δ
python-3.10	93.16% <91.66%> (-0.04%)	⬇️
python-3.11	93.08% <91.66%> (-0.04%)	⬇️
python-3.12	93.20% <91.66%> (-0.04%)	⬇️
server-2024R2	92.25% <62.50%> (-0.63%)	⬇️
server-2025R1	93.08% <91.66%> (-0.04%)	⬇️
server-latest	93.16% <91.66%> (-0.04%)	⬇️

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