fix(schema): remove AddNullable from draft2020_12 settings (#664)

* fix(schema): remove AddNullable from draft2020_12 settings

The `nullable` keyword is an OpenAPI 3.0 extension, not part of
JSON Schema 2020-12. Using AddNullable with draft2020_12 settings
causes validation failures with strict JSON Schema validators.

JSON Schema 2020-12 represents nullable types using:
- {"type": ["string", "null"]} (type array with null)
- {"anyOf": [{"type": "string"}, {"type": "null"}]}

Fixes #663

* test(schema): update complex schema nullable expectation

* test(schema): align macro optional-field expectations with draft2020

crates/rmcp/src/handler/server/common.rs

@@ -23,8 +23,10 @@ pub fn schema_for_type<T: JsonSchema + std::any::Any>() -> Arc<JsonObject> {
         } else {
             // explicitly to align json schema version to official specifications.
             // refer to https://github.com/modelcontextprotocol/modelcontextprotocol/pull/655 for details.
-            let mut settings = SchemaSettings::draft2020_12();
-            settings.transforms = vec![Box::new(schemars::transform::AddNullable::default())];
+            let settings = SchemaSettings::draft2020_12();
+            // Note: AddNullable is intentionally NOT used here because the `nullable` keyword
+            // is an OpenAPI 3.0 extension, not part of JSON Schema 2020-12. Using it would
+            // cause validation failures with strict JSON Schema validators.
             let generator = settings.into_generator();
             let schema = generator.into_root_schema_for::<T>();
             let object = serde_json::to_value(schema).expect("failed to serialize schema");