Omit icons from responses when empty or nil

Previously, the `icons` parameter was always included in responses (as an empty array)
even when not configured. This differs from the response representation in MCP Ruby SDK 0.4.0.

Since `icons` is optional, it should be omitted from responses when unused to reduce
unnecessary context window usage.
https://modelcontextprotocol.io/specification/2025-11-25/schema#implementation

This fix applies to `Tool#to_h`, `Prompt#to_h`, `Resource#to_h`, `ResourceTemplate#to_h`,
and `Server#server_info`.

It doesn't seem critical, but if there are no other issues, it would be good to cut 0.5.1 bugfix release soon.

test/mcp/prompt_test.rb

@@ -195,5 +195,24 @@ module MCP
 
       assert_equal expected, prompt.to_h
     end
+
+    test "#to_h does not have `:icons` key when icons is empty" do
+      prompt = Prompt.define(
+        name: "prompt_without_icons",
+        description: "a prompt without icons",
+      )
+
+      refute prompt.to_h.key?(:icons)
+    end
+
+    test "#to_h does not have `:icons` key when icons is nil" do
+      prompt = Prompt.define(
+        name: "prompt_without_icons",
+        description: "a prompt without icons",
+        icons: nil,
+      )
+
+      refute prompt.to_h.key?(:icons)
+    end
   end
 end