Fix: Preserve domain information when writing GDX files

[MartinBonde]

Summary

• Fixes Domains are not preserved when writing GDX files #3: Domain information was lost when writing GDX files via write_gdx, causing round-tripped symbols to lose their domain associations (e.g. [i] becoming [*])
• Added gdx_symbol_set_domain_x C binding wrapping gdxSymbolSetDomainX from the GDX API
• Each write function (_write_set, _write_parameter, _write_variable, _write_equation) now calls gdxSymbolSetDomainX after gdx_data_write_done to store the relaxed domain information
• Added 5 test cases verifying domain preservation for parameters, variables, equations, multi-dimensional symbols, and GAMS-generated file round-trips

Test plan

• All 115 tests pass (109 existing + 6 new assertions across 5 new testsets)
• Verified parameter domain preserved on round-trip
• Verified variable domain preserved on round-trip
• Verified equation domain preserved on round-trip
• Verified multi-dimensional domain preserved on round-trip
• Verified GAMS-generated file domain preserved on round-trip
• Existing tests unaffected