Validate the src and sizes of an icon against the specification

## Motivation and Context

`MCP::Icon` validated `theme` but accepted anything for `src` and `sizes`: `src:` defaulted to `nil`,
so `MCP::Icon.new.to_h` was `{}`, an icon with no source, and a `sizes` given as a String reached `to_h` unchanged
and serialized as a JSON string where the specification's `Icon` type requires an array of strings.
Nothing on the server side reported either; the server answered every request with 200 and the failure surfaced
in a client, with no reference to the field responsible. The TypeScript SDK types `sizes` as `string[]`
and `src` as required, and the Python SDK's model refuses such an icon at construction, so this SDK was
the one that let it through.

`src` is now a required keyword and must be a non-empty String, `sizes` must be `nil` or an Array of Strings,
and `mime_type` must be `nil` or a String, each refused with `ArgumentError` at construction the way `theme` already was.
The checks stop at the schema's types, as the reference SDKs do: the scheme of a URL and the `WxH` form of a size stay
the caller's to get right. A message names the class of the rejected value, never the value,
since `src` may carry a `data:` URI of any length. Coercing a String into a one-element Array was considered and rejected:
it would hide the caller's mistake that the class exists to surface. The `theme` check let `false` through its truthiness test
and serialized it; it now refuses everything but `nil`, `"light"`, and `"dark"`.

Fixes #562.

## How Has This Been Tested?

New tests in `test/mcp/icon_test.rb`. Against the previous library, an icon without `src` and an icon whose `sizes` is
a String are both constructed and serialized.

## Breaking Changes

`MCP::Icon.new` now raises `ArgumentError` where earlier releases produced an icon the specification's schema rejects:
`src:` is required and must be a non-empty String, so a call without it, or with `nil`, an empty String, or a non-String,
fails; `sizes:` must be `nil` or an Array of Strings, so a String, or an Array holding `nil` or a non-String, fails;
`mime_type:` must be `nil` or a String; `theme:` must be `nil`, `"light"`, or `"dark"`, so `false` fails where it used
to be serialized. Leaving the optional keywords out, or passing `nil` for them, is unchanged.

lib/mcp/icon.rb

@@ -1,13 +1,35 @@
 # frozen_string_literal: true
 
 module MCP
+  # An icon attached to a server, tool, prompt, or resource, per the specification's `Icon` type:
+  # https://modelcontextprotocol.io/specification/2026-07-28/basic#icons
+  #
+  # Each argument is checked against the schema's type, as the TypeScript and Python SDKs check theirs,
+  # so an icon that would serialize into something a client rejects fails here instead. What a value means
+  # is not judged: `src` may be any non-empty `String` (the schema types it as a URI, which an empty `String`
+  # is not, and the specification allows an HTTP/HTTPS URL or a `data:` URI), and a size may be any `String`
+  # (the specification expects `WxH` or `"any"`).
   class Icon
     SUPPORTED_THEMES = ["light", "dark"].freeze
 
     attr_reader :mime_type, :sizes, :src, :theme
 
-    def initialize(mime_type: nil, sizes: nil, src: nil, theme: nil)
-      raise ArgumentError, 'The value of theme must specify "light" or "dark".' if theme && !SUPPORTED_THEMES.include?(theme)
+    def initialize(mime_type: nil, sizes: nil, src:, theme: nil)
+      unless src.is_a?(String) && !src.empty?
+        raise ArgumentError, "The value of src must be a non-empty String (got #{src.class})."
+      end
+
+      unless mime_type.nil? || mime_type.is_a?(String)
+        raise ArgumentError, "The value of mime_type must be a String (got #{mime_type.class})."
+      end
+
+      if (problem = sizes_problem(sizes))
+        raise ArgumentError, "The value of sizes must be an Array of Strings such as [\"48x48\"] or [\"any\"] (#{problem})."
+      end
+
+      unless theme.nil? || SUPPORTED_THEMES.include?(theme)
+        raise ArgumentError, 'The value of theme must specify "light" or "dark".'
+      end
 
       @mime_type = mime_type
       @sizes = sizes
@@ -18,5 +40,16 @@ module MCP
     def to_h
       { mimeType: mime_type, sizes: sizes, src: src, theme: theme }.compact
     end
+
+    private
+
+    def sizes_problem(sizes)
+      return if sizes.nil?
+      return "got #{sizes.class}" unless sizes.is_a?(Array)
+
+      index = sizes.index { |size| !size.is_a?(String) }
+
+      "got #{sizes[index].class} inside the Array" if index
+    end
   end
 end