Fix rb_define_method(singleton_class_of_module, ...) not to change nested module name (#1458)

Fixes #1440
Always track class and singleton class stored to a variable to
`@classes`.
Otherwise, `find_class var_name, class_name` called from `handle_method`
will add a new class.

Before:
<img width="303" height="167" alt="before"
src="https://github.com/user-attachments/assets/fb885b6c-40bf-44c5-8f01-095cfdc75c16"
/>
After:
<img width="303" height="167" alt="after"
src="https://github.com/user-attachments/assets/e0aae152-f46e-4954-8b68-23b74dd3bc90"
/>


Minimal reproduction:
```c
VALUE mFoo = rb_define_module("Foo");
VALUE mBar = rb_define_module_under(mFoo, "Bar");
VALUE mBarS = rb_singleton_class(mBar);
rb_define_method(mBarS, "baz", baz, 0);
```

`RDoc::Parser::C#handle_method` calls `find_class 'mBarS', 'Foo::Bar'`.
`RDoc::Parser::C#find_class` calls `@top_level.add_class
RDoc::NormalClass, 'Foo::Bar'`. This will create `::Bar` for some reason
and `Foo::Bar` disappears from generated document.

Author: tompng

lib/rdoc/parser/c.rb

@@ -1052,10 +1052,13 @@ def handle_method(type, var_name, meth_name, function, param_count,
   # Registers a singleton class +sclass_var+ as a singleton of +class_var+
 
   def handle_singleton(sclass_var, class_var)
-    class_name = @known_classes[class_var]
-
-    @known_classes[sclass_var]     = class_name
-    @singleton_classes[sclass_var] = class_name
+    if (klass = @classes[class_var])
+      @classes[sclass_var] = klass
+    end
+    if (class_name = @known_classes[class_var])
+      @known_classes[sclass_var]     = class_name
+      @singleton_classes[sclass_var] = class_name
+    end
   end
 
   ##

test/rdoc/parser/c_test.rb

@@ -781,6 +781,40 @@ def test_do_methods_singleton_class
     assert               methods.first.singleton
   end
 
+  def test_do_methods_nested_module_singleton_class
+    parser = util_parser <<~EOF
+      VALUE baz(VALUE klass, VALUE year) {
+      }
+      void Init_Foo(void) {
+        VALUE mFoo = rb_define_module("Foo");
+        VALUE mBar = rb_define_module_under(mFoo, "Bar");
+        VALUE mBarS = rb_singleton_class(mBar);
+        rb_define_method(mBarS, "baz", baz, 0);
+      }
+    EOF
+    parser.scan
+
+    klass = parser.classes['mBarS']
+    assert_equal 'Foo::Bar', klass.full_name
+    assert_equal 'Foo::Bar', parser.singleton_classes['mBarS']
+    methods = klass.method_list
+    assert_equal 1,      methods.length
+    assert_equal 'baz',  methods.first.name
+    assert               methods.first.singleton
+  end
+
+  def test_do_singleton_class_undocumentable
+    parser = util_parser <<~EOF
+      void Func(VALUE v) {
+        VALUE k = rb_singleton_class(v);
+        rb_define_method(k, "baz", baz, 0);
+      }
+    EOF
+    parser.scan
+    assert_empty parser.classes
+    assert_empty parser.singleton_classes
+  end
+
   def test_do_missing
     parser = util_parser
 
@@ -1973,7 +2007,8 @@ def test_scan
     expected = {
       @fn => {
         'mM' => 'M',
-        'cC' => 'C', }}
+        'cC' => 'C',
+        'sC' => 'C' }}
     assert_equal expected, @store.c_class_variables
 
     expected = {