Fix constant name of Ractor::IsolationError message

`dest` of `const_decl_path` is `NODE_COLON2` or `NODE_COLON3` in some cases.
For example, `B::C ||= [“Not ” + “shareable”]` passes `NODE_COLON2`
and `::C ||= [“Not ” + “shareable”]` passes `NODE_COLON3`.
This commit fixes `Ractor::IsolationError` message for such case.

```
# shareable_constant_value: literal
::C ||= ["Not " + "shareable"]

# Before
# => cannot assign unshareable object to C (Ractor::IsolationError)

# After
# => cannot assign unshareable object to ::C (Ractor::IsolationError)
```

parse.y

@@ -13639,12 +13639,30 @@ const_decl_path(struct parser_params *p, NODE **dest)
     if (!nd_type_p(n, NODE_CALL)) {
         const YYLTYPE *loc = &n->nd_loc;
         VALUE path;
-        if (RNODE_CDECL(n)->nd_vid) {
-             path = rb_id2str(RNODE_CDECL(n)->nd_vid);
+        switch (nd_type(n)) {
+          case NODE_CDECL:
+            if (RNODE_CDECL(n)->nd_vid) {
+                path = rb_id2str(RNODE_CDECL(n)->nd_vid);
+                goto end;
+            }
+            else {
+                n = RNODE_CDECL(n)->nd_else;
+            }
+            break;
+          case NODE_COLON2:
+            break;
+          case NODE_COLON3:
+            // ::Const
+            path = rb_str_new_cstr("::");
+            rb_str_append(path, rb_id2str(RNODE_COLON3(n)->nd_mid));
+            goto end;
+          default:
+            rb_bug("unexpected node: %s", ruby_node_name(nd_type(n)));
+            UNREACHABLE_RETURN(0);
         }
-        else {
-            n = RNODE_CDECL(n)->nd_else;
-            path = rb_ary_new();
+
+        path = rb_ary_new();
+        if (n) {
             for (; n && nd_type_p(n, NODE_COLON2); n = RNODE_COLON2(n)->nd_head) {
                 rb_ary_push(path, rb_id2str(RNODE_COLON2(n)->nd_mid));
             }
@@ -13662,8 +13680,9 @@ const_decl_path(struct parser_params *p, NODE **dest)
                 rb_ary_push(path, rb_str_new_cstr("..."));
             }
             path = rb_ary_join(rb_ary_reverse(path), rb_str_new_cstr("::"));
-            path = rb_fstring(path);
         }
+      end:
+        path = rb_fstring(path);
         *dest = n = NEW_LIT(path, loc);
         RB_OBJ_WRITTEN(p->ast, Qnil, RNODE_LIT(n)->nd_lit);
     }

test/ruby/test_parse.rb

@@ -1525,6 +1525,40 @@ def test_shareable_constant_value_unshareable_literal
         ::B::C = ["Not " + "shareable"]
       end
     end;
+
+    assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      assert_raise_with_message(Ractor::IsolationError, /unshareable object to ::C/) do
+        # shareable_constant_value: literal
+        ::C ||= ["Not " + "shareable"]
+      end
+    end;
+
+    assert_raise_separately(Ractor::IsolationError, /unshareable object to B::C/,
+                            "#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      # shareable_constant_value: literal
+      B = Class.new
+      B::C ||= ["Not " + "shareable"]
+    end;
+
+    assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      assert_raise_with_message(Ractor::IsolationError, /unshareable object to ::B::C/) do
+        # shareable_constant_value: literal
+        ::B = Class.new
+        ::B::C ||= ["Not " + "shareable"]
+      end
+    end;
+
+    assert_raise_separately(Ractor::IsolationError, /unshareable object to ...::C/,
+                            "#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      # shareable_constant_value: literal
+      B = Class.new
+      def self.expr; B; end
+      expr::C ||= ["Not " + "shareable"]
+    end;
   end
 
   def test_shareable_constant_value_nonliteral