Add missing Title and Icons properties to ResourceLinkBlock

[Copilot]

ResourceLinkBlock was missing Title and Icons properties. Per the 2025-11-25 spec, ResourceLink extends Resource, and Resource extends BaseMetadata, Icons—so ResourceLink inherits both title and icons.

Changes

• ResourceLinkBlock: Added Title (string?) and Icons (IList<Icon>?) properties
• ContentBlock.Converter: Updated custom JSON read/write to handle title and icons for resource_link type, using AOT-compatible source-generated type info for Icon serialization
• Tests: Updated existing roundtrip/minimal tests, added dedicated test for Title and Icons deserialization

var link = new ResourceLinkBlock
{
    Uri = "https://example.com/resource",
    Name = "my-resource",
    Title = "My Resource",
    Icons = [new Icon { Source = "https://example.com/icon.png", MimeType = "image/png" }]
};

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