Implement SEP-973: Add icons and metadata support

[kakooch]

Summary

This PR implements SEP-973 by adding icon support to MCP schema types, enabling clients to visually identify servers, tools, resources, and prompts.

Changes

• New Icon record with src, mimeType, and sizes fields
• Implementation: Added icons and websiteUrl fields
• Tool: Added icons field and updated Builder
• Resource: Added icons field and updated Builder
• Prompt: Added icons field

Icon Type Details

The Icon record supports:

• HTTP/HTTPS URLs for remote icons
• Data URIs for embedded base64-encoded icons (e.g., data:image/png;base64,...)
• Multiple sizes following Web Manifest format (e.g., "48x48", "48x48 96x96", "any")
• Common image MIME types (image/png, image/svg+xml, image/x-icon, etc.)

Example Usage

// Server implementation with icons
var implementation = new Implementation(
    "my-server",
    "My Server",
    "1.0.0",
    List.of(
        new Icon("https://example.com/icon-48.png", "image/png", "48x48"),
        new Icon("https://example.com/icon.svg", "image/svg+xml", "any")
    ),
    "https://example.com"
);

// Tool with icon
var tool = Tool.builder()
    .name("search")
    .description("Search the web")
    .icons(List.of(new Icon("https://example.com/search-icon.png")))
    .build();

Backward Compatibility

All existing constructors remain functional and pass null for the new optional fields. This ensures a seamless upgrade path for existing implementations.

Test Plan

• Build compiles successfully
• Spring Java format validation passes
• Unit tests for Icon record validation
• Serialization/deserialization tests for new fields

Related

• Closes Implement SEP-973: Icons and metadata support #532
• Implements SEP-973

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