feat(mcp-annotations): add SEP-1865 MCP Apps support

[apappascs]

Summary

Add SEP-1865 (MCP Apps) support to the mcp-annotations module, enabling Spring AI MCP servers to expose interactive UI widgets alongside their tools.

What's included

• @McpTool extensions — resourceUri, visibility, and csp fields to link tools to UI resources and control visibility (model-only, app-only, or both)
• Visibility enum — MODEL and APP visibility scopes
• @McpCsp annotation — declare Content Security Policy domains (connectDomains, resourceDomains, redirectDomains) for widget sandboxing
• McpAppResult — convenience return type that splits text → content[] (for the LLM) and structuredContent (for the widget UI)
• MetaUtils — buildUiMeta() and mergeMeta() helpers to construct _meta.ui blocks per the SEP-1865 spec
• Tool provider updates — all 4 providers (sync/async × stateful/stateless) now merge UI metadata into tool definitions
• ResourceAdapter — validates ui:// URIs and text/html;profile=mcp-app MIME type for MCP App resources
• Comprehensive tests — McpAppResultTests, ResourceAdapterTests, MetaUtilsTest, SyncMcpToolProviderTests

Example usage

@McpTool(
    name = "my_tool",
    description = "...",
    resourceUri = "ui://my-server/widget.html",
    visibility = { Visibility.MODEL, Visibility.APP }
)
public McpAppResult myTool(@McpToolParam(description = "...") String input) {
    return McpAppResult.of(
        "Text for the LLM",
        Map.of("key", "structured data for the widget")
    );
}

Context

SEP-1865 defines how MCP servers expose interactive UIs (widgets) that render inside AI chat clients (ChatGPT, Claude, etc.). This PR adds first-class annotation support so Spring AI developers can build MCP Apps with the same @McpTool / @McpResource patterns they already use.

Test plan

• McpAppResultTests — verifies text/structuredContent split
• ResourceAdapterTests — validates ui:// URI and MIME type handling
• MetaUtilsTest — tests meta construction and merging
• SyncMcpToolProviderTests — verifies tool providers inject UI metadata
• Manual end-to-end testing with MCPJam Inspector and ChatGPT

[apappascs]

Reference Links for Reviewers ( @tzolov )

• SEP-1865 specification: https://github.com/modelcontextprotocol/modelcontextprotocol/blob/main/seps/1865-mcp-apps-interactive-user-interfaces-for-mcp.md
• Reference implementation in ext-apps SDK (3,864 lines): spec: Add tool registration for Apps, to be called by Host (WebMCP-style!) modelcontextprotocol/ext-apps#72