Cache helpers over websocket

The same distributed cache that backs HTTP POST /api/cache/* routes is available on the session channel for connected clients.

await client.connect();

await client.setData("users", "alice", { role: "admin" }, { ttlMs: 30_000 });
const value = await client.getData("users", "alice");
await client.updateData("users", "alice", { role: "superadmin" });
const deleted = await client.deleteData("users", "alice");

TTL values must be positive integers within the server-configured maximum. When shards are unavailable or the cache is rehydrating, calls fail with the same string reason codes documented for HTTP (including update-specific reasons such as not_found when the key does not exist).