Add edge case regression tests for --generate-function-body const handling

[tautschnig]

The existing tests provided basic const handling coverage, but several complex scenarios were not comprehensively tested. This PR addresses the following edge cases:

1. Triple pointer const handling - int * const ** (const at middle level)
2. Double const qualification - const int * const (both pointer and data const)
3. Array of pointers to const - const int ** / const int *arr[]
4. Const function pointers - func_ptr_t * const
5. Nested struct const pointers - Mixed const/non-const in nested structures
6. Combined qualifiers - volatile const int * (volatile + const interaction)
7. Const array parameters - const int[] (array parameter const protection)
8. Pointer to const struct - const struct test_struct * (struct-level const)
9. Systematic const variations - Comprehensive test of const at each pointer level

Co-authored-by: Kiro autonomous agent

Fixes: #1948

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