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/tests/test_complex_schema.rs

@@ -80,8 +80,10 @@ fn expected_schema() -> serde_json::Value {
           "type": "array"
         },
         "system": {
-          "nullable": true,
-          "type": "string"
+          "type": [
+            "string",
+            "null"
+          ]
         }
       },
       "required": [