Extract MCP::Tool::Annotations unit tests

The tests related to `MCP::Tool::Annotations` will be extracted from the `MCP::ToolTest` test class
into a dedicated `MCP::Tool::AnnotationsTest` unit test class as a testing pair.

This improves test clarity, file level test independence, and file level execution speed.
It also ensures consistent placement as a testing pair under `MCP::Tool::*`:

```console
$ ls lib/mcp/tool/
annotations.rb  input_schema.rb  response.rb
$ ls test/mcp/tool/
annotations_test.rb  input_schema_test.rb  response_test.rb
```

test/mcp/tool/annotations_test.rb

@@ -0,0 +1,75 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+module MCP
+  class Tool
+    class AnnotationsTest < ActiveSupport::TestCase
+      test "Tool::Annotations initializes with all properties" do
+        annotations = Tool::Annotations.new(
+          title: "Test Tool",
+          read_only_hint: true,
+          destructive_hint: false,
+          idempotent_hint: true,
+          open_world_hint: false,
+        )
+
+        assert_equal "Test Tool", annotations.title
+        assert annotations.read_only_hint
+        refute annotations.destructive_hint
+        assert annotations.idempotent_hint
+        refute annotations.open_world_hint
+      end
+
+      test "Tool::Annotations initializes with partial properties" do
+        annotations = Tool::Annotations.new(
+          title: "Test Tool",
+          read_only_hint: true,
+        )
+
+        assert_equal "Test Tool", annotations.title
+        assert annotations.read_only_hint
+        assert_nil annotations.destructive_hint
+        assert_nil annotations.idempotent_hint
+        assert_nil annotations.open_world_hint
+      end
+
+      test "Tool::Annotations#to_h omits nil values" do
+        annotations = Tool::Annotations.new(
+          title: "Test Tool",
+          read_only_hint: true,
+        )
+
+        expected = {
+          title: "Test Tool",
+          readOnlyHint: true,
+        }
+        assert_equal expected, annotations.to_h
+      end
+
+      test "Tool::Annotations#to_h handles all properties" do
+        annotations = Tool::Annotations.new(
+          title: "Test Tool",
+          read_only_hint: true,
+          destructive_hint: false,
+          idempotent_hint: true,
+          open_world_hint: false,
+        )
+
+        expected = {
+          title: "Test Tool",
+          readOnlyHint: true,
+          destructiveHint: false,
+          idempotentHint: true,
+          openWorldHint: false,
+        }
+        assert_equal expected, annotations.to_h
+      end
+
+      test "Tool::Annotations#to_h returns empty hash when all values are nil" do
+        annotations = Tool::Annotations.new
+        assert_empty annotations.to_h
+      end
+    end
+  end
+end

test/mcp/tool_test.rb

@@ -145,73 +145,6 @@ module MCP
       assert_equal({ readOnlyHint: true, title: "Mock Tool" }, tool.annotations_value.to_h)
     end
 
-    # Tests for Tool::Annotations class
-    test "Tool::Annotations initializes with all properties" do
-      annotations = Tool::Annotations.new(
-        title: "Test Tool",
-        read_only_hint: true,
-        destructive_hint: false,
-        idempotent_hint: true,
-        open_world_hint: false,
-      )
-
-      assert_equal "Test Tool", annotations.title
-      assert annotations.read_only_hint
-      refute annotations.destructive_hint
-      assert annotations.idempotent_hint
-      refute annotations.open_world_hint
-    end
-
-    test "Tool::Annotations initializes with partial properties" do
-      annotations = Tool::Annotations.new(
-        title: "Test Tool",
-        read_only_hint: true,
-      )
-
-      assert_equal "Test Tool", annotations.title
-      assert annotations.read_only_hint
-      assert_nil annotations.destructive_hint
-      assert_nil annotations.idempotent_hint
-      assert_nil annotations.open_world_hint
-    end
-
-    test "Tool::Annotations#to_h omits nil values" do
-      annotations = Tool::Annotations.new(
-        title: "Test Tool",
-        read_only_hint: true,
-      )
-
-      expected = {
-        title: "Test Tool",
-        readOnlyHint: true,
-      }
-      assert_equal expected, annotations.to_h
-    end
-
-    test "Tool::Annotations#to_h handles all properties" do
-      annotations = Tool::Annotations.new(
-        title: "Test Tool",
-        read_only_hint: true,
-        destructive_hint: false,
-        idempotent_hint: true,
-        open_world_hint: false,
-      )
-
-      expected = {
-        title: "Test Tool",
-        readOnlyHint: true,
-        destructiveHint: false,
-        idempotentHint: true,
-        openWorldHint: false,
-      }
-      assert_equal expected, annotations.to_h
-    end
-
-    test "Tool::Annotations#to_h returns empty hash when all values are nil" do
-      annotations = Tool::Annotations.new
-      assert_empty annotations.to_h
-    end
-
     test "Tool class method annotations can be set and retrieved" do
       class AnnotationsTestTool < Tool
         tool_name "annotations_test"