Skip recursive-extension StackError spec on JRuby

The MessagePack::StackError behavior verified by this spec is specific to the
C extension, which caps nesting at MSGPACK_UNPACKER_STACK_CAPACITY. The Java
(JRuby) decoder has no equivalent guard and recurses until the JVM stack
overflows, so the spec cannot assert MessagePack::StackError there.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: Watson1978

spec/factory_spec.rb

@@ -663,6 +663,11 @@ class << Symbol
     end
 
     it 'raises StackError instead of crashing on deeply nested recursive extensions' do
+      # The C extension caps nesting at MSGPACK_UNPACKER_STACK_CAPACITY and raises
+      # MessagePack::StackError. The Java (JRuby) decoder has no such guard and
+      # recurses until the JVM stack overflows, so this behavior is CRuby-specific.
+      skip if IS_JRUBY
+
       recursive_type = Struct.new(:payload)
       factory = MessagePack::Factory.new
       factory.register_type(0x01,