feat: update default is_internal() to return true and clean up overrides

Co-authored-by: aider (openai/andrew/openrouter/deepseek/deepseek-chat-v3.1) <aider@aider.chat>

src/meta_plugin/binary.rs

@@ -65,9 +65,6 @@ impl BinaryMetaPlugin {
 }
 
 impl MetaPlugin for BinaryMetaPlugin {
-    fn is_internal(&self) -> bool {
-        true
-    }
 
     fn update(&mut self, data: &[u8]) -> MetaPluginResponse {
         // Calculate how much data we can still accept