docs: update client capabilities for MCP 2026-07-28

[dawNotPoi]

Summary

• document the MCP 2026-07-28 request-scoped UI capability under _meta["io.modelcontextprotocol/clientCapabilities"].extensions on every request
• preserve the MCP 2025-11-25 initialize compatibility example for earlier protocol versions
• distinguish client-to-server capability advertisement from the View-to-Host ui/initialize handshake and its params.appCapabilities layer
• update server guidance to evaluate capabilities per request and degrade gracefully when UI support is absent

Validation

• npm run build
• npm test (373 passed, 2 skipped)
• npm run prettier
• npm run build:all (pre-commit hook)
• parsed the added JSON examples successfully
• verified the linked MCP 2026-07-28 metadata page returns HTTP 200
• git diff --check

Fixes #742