feat: get sandbox by metadata filter

[OndrejDrapalik]

What

Adds a Get By selector to the Sandbox → Get operation:

• ID (default) — unchanged existing behavior
• Metadata — retrieve the first sandbox matching a metadata filter (JSON object, keys combined with AND)

Why

Finding a persistent sandbox by metadata currently requires Get Many + a Code node:

const match = $input.all().find(
  (i) => i.json?.metadata?.purpose === 'my-agent' && i.json.state !== 'error'
);

This breaks once the sandbox falls outside the Get Many limit, and it's a lot of ceremony for "give me my sandbox". The API already supports server-side filtering via the metadata query parameter of GET /v2/sandboxes (same filter the JS SDK exposes as Sandbox.list({ query: { metadata } })) — the node just didn't expose it.

How

• listSandboxes in client.ts now takes an options object ({ limit, metadata }) and serializes the metadata filter as URL-encoded key=value pairs joined with &, matching the SDK's encoding (verified against the live API).
• Get-by-metadata lists with limit: 1 + the filter, then fetches the matched sandbox by ID, so the output shape is identical to Get by ID.
• No match / empty filter → clear NodeOperationError.

Testing

• pnpm test — 16/16 (5 new: get by ID routing, get by metadata, no match, empty filter, getMany limit passthrough)
• pnpm typecheck, pnpm lint, pnpm build — all clean
• Metadata query format verified against the production API (?metadata=purpose%3D... returns the exact matching sandbox; unknown value returns [])