[DSLX Fuzz testing] Unify fuzz test domain validation via constexpr value evaluation to support derived and nested struct domains.

[copybara-service]

[DSLX Fuzz testing] Unify fuzz test domain validation via constexpr value evaluation to support derived and nested struct domains.

Moved fuzz test domain validation to its own class file.

To support derived and nested struct domains, this CL:

• Shifts the domain validation paradigm: instead of validating the AST structure before evaluation, ValidateFuzzTestDomain now evaluates the constexpr domain expression to an InterpValue and recursively validates this evaluated value against the parameter's concrete type.
• Migrates recursive validation of struct domains to the new post-evaluation path, validating evaluated domain tuple elements against StructType members.
• Relaxes module type-checking constraints during domain struct initialization to allow assigning range and array domains to fields, relying on the post-evaluation validation to ensure type safety.
• Fixes a bug where empty tuple domains () (which represent an arbitrary domain) were incorrectly rejected for non-tuple/non-struct parameters. Empty tuples are now correctly permitted as arbitrary domains for all parameter types.