Implement SEP-973: Icons and metadata support for Implementations, Resources, Tools, and Prompts

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SEP-973: Icons and metadata support implementation

This PR implements SEP-973 which adds support for icons and additional metadata for Implementations, Resources, Tools, and Prompts.

Implementation Plan

• Review and understand SEP-973 specification requirements
• Understand current codebase structure and build system
• Create Icon class with src, mimeType, and sizes properties
• Add Icons property to Implementation class
• Add WebsiteUrl property to Implementation class
• Add Icons property to Resource class
• Add Icons property to Tool class
• Add Icons property to Prompt class
• Create comprehensive tests for Icon validation and serialization
• Create tests for all updated classes with icon support
• Fix property accessor consistency across classes
• Address code review feedback and improve test patterns
• Validate all changes work with existing codebase (requires .NET 9 SDK)
• Test building and ensure backward compatibility

Recent Changes (Addressing Code Review Feedback)

• Removed documentation files: Removed SEP-973-IMPLEMENTATION.md and SEP-973-EXAMPLES.md as they shouldn't be checked in
• Enhanced test patterns: Updated all test classes to follow existing patterns with round-trip serialization validation
• Added JsonException tests: Added tests that verify proper exceptions are thrown for invalid JSON payloads
• Split test files: Separated combined test classes into individual files (ResourceTests.cs, PromptTests.cs)
• Renamed test classes: Changed ToolIconTests to ToolTests and made tests less icon-centric
• Improved test coverage: Tests now focus on overall functionality rather than just icon-specific behavior

Key Changes Implemented

1. New Icon Class (Icon.cs)

• Required: Src property for URI pointing to icon resource
• Optional: MimeType and Sizes properties
• Supports HTTP/HTTPS URLs and data URIs
• Comprehensive security documentation per SEP-973
• Uses init-only properties for immutability

2. Enhanced Classes with Icon Support

• Implementation: Added Icons and WebsiteUrl properties
• Resource: Added Icons property
• Tool: Added Icons property
• Prompt: Added Icons property

3. Specification Compliance

✅ Matches TypeScript schema exactly
✅ Correct JSON property names (icons, websiteUrl, src, mimeType, sizes)
✅ All new properties are optional for backward compatibility
✅ Supports multiple icons per entity
✅ Documents required (PNG, JPEG) and recommended (SVG, WebP) MIME types
✅ Includes security considerations for SVG and URI validation

4. Comprehensive Test Coverage

• IconTests.cs: Icon serialization, round-trip validation, and JsonException tests
• ImplementationTests.cs: Full Implementation testing with required property validation
• ToolTests.cs: Tool functionality testing including new icon support
• ResourceTests.cs: Resource serialization and property validation
• PromptTests.cs: Prompt functionality testing
• All tests follow existing repository patterns with round-trip validation
• Tests validate JsonException behavior for invalid JSON
• Tests cover all properties, not just icon-specific functionality

Technical Details

• Uses IList<Icon>? for consistency with existing collection patterns
• Proper JSON serialization with JsonPropertyName attributes
• Follows existing codebase conventions and patterns
• All optional properties use nullable types
• Test classes follow established naming and structure patterns

Security Features

Implementation includes documentation for:

• URI validation and trusted domain requirements
• SVG security precautions (executable JavaScript content)
• Resource exhaustion protection recommendations
• MIME type validation guidelines

Note: Build validation requires .NET 9 SDK. The implementation follows established patterns and should compile correctly with the appropriate SDK version.

Fixes #753.

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