Validate the src and sizes of an icon against the specification

[koic]

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.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed