feat(devbox): add devbox.onEvict eviction notifications (SSE)

[jrvb-rl]

Description

Follow-on to the server-side flex eviction-notification work (monorepo #10325 and its successor SSE endpoint). Adds the object-oriented client surface for eviction notifications.

New API: devbox.onEvict(callback) (and devbox.cancelOnEvict()) on the Devbox wrapper.

How it works (internal state management):

• A per-client EvictionMonitor (src/sdk/eviction.ts) holds the interest set: Map<devbox_id, callback>. Monitors are looked up via a WeakMap<Runloop, EvictionMonitor> (getEvictionMonitor), so every Devbox built from the same generated client shares one monitor and thus one SSE connection. No Devbox/factory signatures changed.
• The first onEvict across any devbox opens a single account-wide devboxes.watchEvictions() SSE stream via a background for await loop.
• On each event: the id is matched against the interest set; unknown ids are discarded. The devbox is removed before its callback runs, so it fires at most once even if the server repeats a notification.
• When the interest set empties, the stream's AbortController is aborted and the stream closed. Re-registering re-opens it.
• The server replays all currently-pending evictions on connect, so late registrants are still notified.

Dependency / status

⚠️ Draft. This depends on the Stainless-generated devboxes.watchEvictions method and the DevboxEvictionEvent type (devbox_id, eviction_deadline_ms), which are generated downstream once the server endpoint lands. Those symbols are imported by assumed name from ../resources/devboxes/devboxes and will be reconciled once generation lands. No generated code was modified.

Note: jest.config.objects.js enforces 100% function coverage over src/sdk, so the object-oriented smoke test for onEvict/cancelOnEvict (plus a mocked tests/objects unit test) is required and will be added when the endpoint is available to exercise.

Testing

Deferred until the generated endpoint exists (see above).

🤖 Generated with Claude Code

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[jrvb-rl]

Reconciled with the endpoint as landed:

• callback is now callback(devbox, evictionDeadlineMs) (the Devbox object + Unix-ms deadline)
• uses the real generated type DevboxEvictionEventView

Ordering: depends on runloopai/runloop#10404, which marks watch_evictions as streaming in openapi.stainless.yml so Stainless generates devboxes.watchEvictions() -> Stream<DevboxEvictionEventView>. Once that merges and Stainless regenerates (the .stats.yml bump lands the method + type), this rebases green.