fix(rmcp): generate elicitation $schema as optional string in schemars

Use schemars(with = "String") on ElicitationSchema::$schema so schemars generates "type": "string" instead of ["string", "null"].

Author: latent-9

crates/rmcp/src/model/elicitation_schema.rs

@@ -1121,7 +1121,8 @@ pub struct ElicitationSchema {
     /// The 2025-11-25 protocol revision allows a `requestedSchema` to declare its
     /// dialect. It is preserved verbatim so a declared dialect survives a
     /// decode/re-encode round-trip instead of being silently dropped.
-    #[serde(rename = "$schema", skip_serializing_if = "Option::is_none")]
+    #[serde(rename = "$schema", default, skip_serializing_if = "Option::is_none")]
+    #[cfg_attr(feature = "schemars", schemars(with = "String"))]
     pub schema: Option<Cow<'static, str>>,
 
     /// Always "object" for elicitation schemas
@@ -1152,7 +1153,7 @@ pub struct ElicitationSchema {
 #[derive(Deserialize, Serialize)]
 #[serde(rename_all = "camelCase")]
 struct ElicitationSchemaWire {
-    #[serde(rename = "$schema", skip_serializing_if = "Option::is_none")]
+    #[serde(rename = "$schema", default, skip_serializing_if = "Option::is_none")]
     schema: Option<Cow<'static, str>>,
     #[serde(rename = "type")]
     type_: ObjectTypeConst,

crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema.json

@@ -965,10 +965,7 @@
       "properties": {
         "$schema": {
           "description": "Optional JSON Schema dialect identifier (the `$schema` keyword).\n\nThe 2025-11-25 protocol revision allows a `requestedSchema` to declare its\ndialect. It is preserved verbatim so a declared dialect survives a\ndecode/re-encode round-trip instead of being silently dropped.",
-          "type": [
-            "string",
-            "null"
-          ]
+          "type": "string"
         },
         "description": {
           "description": "Optional description of what this schema represents",