Allow $ref in Tool::Schema for protocol version 2025-11-25

`Tool::Schema` rejected `$ref` with an `ArgumentError`.
Earlier MCP spec versions (2024-11-05 through 2025-06-18) did not define
a JSON Schema dialect or reference the `$ref` keyword. `inputSchema` was described
only as "JSON Schema defining expected parameters" with no further constraints:

- https://modelcontextprotocol.io/specification/2024-11-05/server/tools
- https://modelcontextprotocol.io/specification/2025-06-18/server/tools

The MCP spec 2025-11-25 introduced a "JSON Schema Usage" section that adopts
JSON Schema 2020-12 as the default dialect. Since `$ref` is a core keyword in 2020-12,
it is now allowed for protocol version 2025-11-25 and later.

- https://modelcontextprotocol.io/specification/2025-11-25/basic#json-schema-usage
- https://modelcontextprotocol.io/specification/2025-11-25/server/tools
- https://json-schema.org/draft/2020-12/release-notes

For backward compatibility with older protocol versions (2025-06-18 and earlier),
`Server#validate!` continues to raise `ArgumentError` when a tool input schema contains `$ref`.
This preserves the existing behavior for servers targeting those spec versions.

test/mcp/tool/output_schema_test.rb

@@ -82,16 +82,41 @@ module MCP
         end
       end
 
-      test "rejects schemas with $ref references" do
-        assert_raises(ArgumentError) do
-          OutputSchema.new(properties: { foo: { "$ref" => "#/definitions/bar" } }, required: ["foo"])
-        end
+      test "accepts schemas with $ref references" do
+        schema = OutputSchema.new(
+          properties: {
+            foo: { type: "string" },
+          },
+          definitions: {
+            bar: { type: "string" },
+          },
+          required: ["foo"],
+        )
+        assert_includes schema.to_h.keys, :definitions
       end
 
-      test "rejects schemas with symbol $ref references" do
-        assert_raises(ArgumentError) do
-          OutputSchema.new(properties: { foo: { :$ref => "#/definitions/bar" } }, required: ["foo"])
-        end
+      test "accepts schemas with $ref string key and includes $ref in to_h" do
+        schema = OutputSchema.new({
+          "properties" => {
+            "foo" => { "$ref" => "#/definitions/bar" },
+          },
+          "definitions" => {
+            "bar" => { "type" => "string" },
+          },
+        })
+        assert_equal "#/definitions/bar", schema.to_h[:properties][:foo][:$ref]
+      end
+
+      test "accepts schemas with $ref symbol key and includes $ref in to_h" do
+        schema = OutputSchema.new({
+          properties: {
+            foo: { :$ref => "#/definitions/bar" },
+          },
+          definitions: {
+            bar: { type: "string" },
+          },
+        })
+        assert_equal "#/definitions/bar", schema.to_h[:properties][:foo][:$ref]
       end
 
       test "== compares two output schemas with the same properties and required fields" do