Add tests for Ice constants and default values

[bernardnormier]

This PR depends on zeroc-ice/ice#5282

[Copilot]

Pull request overview

Adds new generator baseline tests to validate Slice/Ice constant emission and default value initialization behaviors in the generated C# code.

Changes:

• Introduces a new ConstTests Slice file + NUnit test suite to validate primitive, string (escape sequences), and enum constants (including aliases).
• Introduces a new DefaultValueTests Slice file + NUnit test suite to validate default values for structs, classes, and exceptions (including defaults sourced from constants).
• Covers qualification variations for enums/constants (::Fully::Qualified, module-qualified, and unqualified).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
tests/IceRpc.Ice.Generator.Base.Tests/ConstTests.ice	Defines enums + constants (including aliases) used to validate constant mapping.
tests/IceRpc.Ice.Generator.Base.Tests/ConstTests.cs	NUnit tests verifying generated constant values and escape sequence preservation.
tests/IceRpc.Ice.Generator.Base.Tests/DefaultValueTests.ice	Defines structs/classes/exceptions with explicit defaults (literal and const-based).
tests/IceRpc.Ice.Generator.Base.Tests/DefaultValueTests.cs	NUnit tests asserting generated default-value initialization semantics.