Add a feature tree

[greschd]

Add helper functions to either print a tree of ACP objects, or
print it. In the documentation, show the tree structure, and
explain how it affects the access and creation of objects.

To implement the feature tree, record metadata when creating
mapping properties (value type, and read-only status), and use
a separate _exposed_grpc_property subclass to indicate it is
a mapping.

Other changes:

• change the default look of the model printer, to include lines
  connecting the nodes

25R2 hardening item 11.

[codecov]

Codecov Report

Attention: Patch coverage is 95.74468% with 2 lines in your changes missing coverage. Please review.

Project coverage is 94.92%. Comparing base (4119762) to head (280f288).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ansys/acp/core/_model_printer.py	95.45%	1 Missing ⚠️
src/ansys/acp/core/extras/feature_tree.py	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #821      +/-   ##
==========================================
+ Coverage   94.91%   94.92%   +0.01%     
==========================================
  Files         109      110       +1     
  Lines        5737     5771      +34     
  Branches      328      333       +5     
==========================================
+ Hits         5445     5478      +33     
- Misses        291      292       +1     
  Partials        1        1              

Flag	Coverage Δ
direct-launch	91.45% <95.74%> (+0.03%)	⬆️
minimum-deps	93.09% <95.74%> (+0.02%)	⬆️
python-3.10	94.14% <95.74%> (+0.01%)	⬆️
python-3.11	94.07% <95.74%> (+0.01%)	⬆️
python-3.12	94.28% <95.74%> (+0.01%)	⬆️
server-2024R2	90.66% <95.74%> (+0.03%)	⬆️
server-2025R1	94.02% <95.74%> (+0.01%)	⬆️
server-2025R2	94.07% <95.74%> (+0.01%)	⬆️
server-latest	94.71% <95.74%> (+0.01%)	⬆️

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[greschd]

The check-vulnerabilities action is failing also on main; it appears to be a problem with the action itself.