Fix AttributeError masking dropped AEDT connection in solution_type fallback

[agu2347]

Bug

Reported in #8020. design_solutions.py falls back to self._design_type.default_solution when GetSolutionType() raises, but every constant class in generic/aedt_constants.py (HfssConstants, Q3dConstants, IcepakConstants, Maxwell3dConstants, CircuitConstants, etc.) only defines solution_default:

python -c "from ansys.aedt.core.generic.aedt_constants import HfssConstants as H; print(hasattr(H, 'solution_default'), hasattr(H, 'default_solution'))"
True False

The fallback therefore raises AttributeError itself. Because this fallback runs exactly when the gRPC connection to AEDT has broken, the user sees 'HfssConstants' object has no attribute 'default_solution' instead of a connection error, which points debugging at design types instead of the actual failure.

DesignSolution.solution_type's getter already uses the correct solution_default spelling; the setter (value is None branch) and both of HFSSDesignSolution.solution_type's getter and setter still used default_solution. On current main this affects 9 call sites in src/ansys/aedt/core/application/design_solutions.py, all identical in shape (self._design_type.default_solution).

Fix

Rename all 9 remaining default_solution accesses to solution_default, matching every other call site in the file and the actual attribute the constant classes define. No changes needed to the constants themselves.

Testing

No AEDT installation or license is required to reproduce or verify this — the bug is a pure attribute-name mismatch between two Python modules.

Added tests/unit/test_design_solutions.py:

• test_constants_only_define_solution_default — confirms the constant classes define solution_default and not default_solution.
• test_design_solution_getter_falls_back_on_connection_error — control, exercises the getter's already-correct branch.
• test_design_solution_setter_falls_back_on_connection_error — regression test for the setter's value is None branch, using a fake ODesign whose GetSolutionType() raises to simulate a dropped connection.
• test_design_solution_setter_no_odesign_falls_back — regression test for the no-_odesign fallback branch.
• test_hfss_design_solution_getter_falls_back_on_connection_error — regression test for HFSSDesignSolution's getter.

Verified all 3 regression tests reproduce the exact reported AttributeError against the unfixed code (reverted design_solutions.py to the current main version, reran — 3 failed with AttributeError: type object 'HfssConstants'/'Q3dConstants' has no attribute 'default_solution', 2 passed as controls), then reapplied the fix and reran — all 5 pass.

Fixes #8020

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