docs: correct stable-spec app initialization example

[Michael-WhiteCapData]

Summary

• change the stable specification transport example from the base MCP initialize method to the Apps ui/initialize method
• replace clientInfo and capabilities with the schema-defined appInfo and appCapabilities fields

This makes the example agree with McpUiInitializeRequest and the lifecycle/capabilities sections in the same specification. Implementations copied from the previous example produced a schema-invalid request and never completed the iframe handshake.

Closes #634.

Verification

• npm test — 438 passed, 2 platform-specific skipped
• npm run build
• git diff --check