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.

lib/mcp/tool.rb

@@ -21,7 +21,7 @@ module MCP
           name: name_value,
           title: title_value,
           description: description_value,
-          icons: icons&.map(&:to_h),
+          icons: icons_value&.then { |icons| icons.empty? ? nil : icons.map(&:to_h) },
           inputSchema: input_schema_value.to_h,
           outputSchema: @output_schema_value&.to_h,
           annotations: annotations_value&.to_h,