fmt: fix formatting fixed array size of struct member (#22815)

Author: felipensp

vlib/v/ast/types.v

@@ -1421,6 +1421,9 @@ pub fn (t &Table) type_to_str_using_aliases(typ Type, import_aliases map[string]
 			elem_str := t.type_to_str_using_aliases(info.elem_type, import_aliases)
 			if info.size_expr is EmptyExpr {
 				res = '[${info.size}]${elem_str}'
+			} else if info.size_expr is Ident {
+				size_str := t.shorten_user_defined_typenames(info.size_expr.name, import_aliases)
+				res = '[${size_str}]${elem_str}'
 			} else {
 				res = '[${info.size_expr}]${elem_str}'
 			}

vlib/v/fmt/tests/struct_fixed_array_ident_keep.vv

@@ -0,0 +1,11 @@
+import crypto.ed25519
+
+struct SSHClientHello {
+	version    u8
+	public_key [ed25519.public_key_size]u8
+}
+
+fn main() {
+	a := SSHClientHello{}
+	assert a.public_key.len == ed25519.public_key_size
+}