[Design] Apply an RFC 6902 JSON Patch to an OctoPerf Virtual User's full entity (metadata + recursive `children` action tree). The patch is a JSON string holding an array of operations `[{"op":"add|remove|replace|move|copy|test", "path":"/json/pointer", "value":<json>}, ...]`. Paths use RFC 6901 JSON Pointer (`/children/0/url`, `/name`, …). Polymorphic nodes (actions, extractors, assertions, …) must keep their `@type` discriminator; consult `octoperf://schema/vu` for the per-subtype required fields (also served as JSON at `/mcp/public/schema/vu.json` on this server's origin for clients that can't read MCP resources). The server fetches the current VU, applies the patch on its JSON representation, re-deserializes via Jackson (round-trip validation — invalid schema is rejected) and persists. Returns the updated VirtualUser. Use `get_virtual_user` first to read the current tree and compute precise paths.