GDScript: Fix incorrect default values ​​in _make_arguments_hint()

[dalexeev]

• Fixes Code autocomplete text for default constructed Callable is incorrect (extra "null" string) #94658.

1. Fix concatenation of the function name with reduced value.

godot/modules/gdscript/gdscript_editor.cpp

Lines 796 to 802 in 8e36f98

	case GDScriptParser::Node::CALL: {
	const GDScriptParser::CallNode *call = static_cast<const GDScriptParser::CallNode *>(par->initializer);
	if (call->is_constant && call->reduced) {
	def_val = call->function_name.operator String() + call->reduced_value.operator String();
	} else {
	def_val = call->function_name.operator String() + (call->arguments.is_empty() ? "()" : "(...)");
	}

2. Use Variant::get_construct_string() instead of Variant::operator String() for consistency with native hints and GDScriptDocGen.

godot/modules/gdscript/gdscript_editor.cpp

Lines 728 to 730 in 8e36f98

	if (i - def_args >= 0) {
	arghint += String(" = ") + p_info.default_arguments[i - def_args].get_construct_string();
	}

godot/modules/gdscript/editor/gdscript_docgen.cpp

Lines 227 to 228 in 8e36f98

	default:
	return p_variant.get_construct_string();

3. Fix GDScriptParser::Node::SUBSCRIPT case. For enum values ​​the logic has been corrected; for other types it has been discarded since the previous assumptions are incorrect.

[HolonProduction]

Have tested it locally and it indeed fixes the issue. Code makes sense as well.

The usage of get_constructor_string also fixes quotes around reduced string values not showing properly. Though I doubt there is an issue about it, since we only show the reduced value in the subscript case, speaking of which, maybe you could copy this is_constant && reduced condition to the IDENTIFIER case, to avoid a one line PR for that in the future.

Also I think it would be nice to have some unit tests for this, the completion test runner can check the argument hint. But given that there aren't any tests at all for argument hints, we could also just merge the fix as is and I'll work on a complete set of tests for argument hints.

[dalexeev]

maybe you could copy this is_constant && reduced condition to the IDENTIFIER case

Actually, it will make readability worse. Instead of my_param = MY_CONST you will get my_param = 123. This is inconsistent with GDScriptDocGen, but I prefer not to change it within this PR.

is_constant && reduced

TODO: Change this to just is_constant.

Also I think it would be nice to have some unit tests for this, the completion test runner can check the argument hint. But given that there aren't any tests at all for argument hints, we could also just merge the fix as is and I'll work on a complete set of tests for argument hints.

Agree, let's do it in a separate PR.

[akien-mga]

Thanks!