feat(capabilities): add extensions field for SEP-1724

[aharvard]

This PR adds support for MCP extension capabilities in both ClientCapabilities and ServerCapabilities structs, as specified in SEP-1724.

Changes

• Add ExtensionCapabilities type alias (BTreeMap<String, JsonObject>)
• Add extensions field to ClientCapabilities struct
• Add extensions field to ServerCapabilities struct
• Update builder macros and impl blocks for both structs
• Add comprehensive tests for extension capabilities
• Update JSON schema test fixtures

Usage

This enables clients to advertise extension support during initialize:

let mut extensions = ExtensionCapabilities::new();
extensions.insert(
    "io.modelcontextprotocol/ui".to_string(),
    serde_json::from_value(json!({
        "mimeTypes": ["text/html;profile=mcp-app"]
    })).unwrap(),
);

let capabilities = ClientCapabilities::builder()
    .enable_extensions_with(extensions)
    .enable_sampling()
    .build();

Which serializes to:

{
  "capabilities": {
    "extensions": {
      "io.modelcontextprotocol/ui": {
        "mimeTypes": ["text/html;profile=mcp-app"]
      }
    }
  }
}

Related

• Closes Implement SEP-1724: Extensions #530
• Implements MCP Apps Extension Spec