prewarm reusable env

[dansola]

Motivation

When a reusable TaskEnvironment is first invoked, the backend lazily spawns its actor pool — pulling images, running init containers, starting workers. Subsequent tasks land on the warm pool, but the first one pays the full cold start. Users who know they'll hit a reusable env later in a workflow have no way to ask the SDK to start that pool earlier; today they have to author a hand-rolled dummy task and fire it from the driver. SE-760 adds a first-class API for it.

Summary

Adds TaskEnvironment.prewarm() (async) and TaskEnvironment.prewarm_sync() (matching download_sync / load_sync convention). Calling either submits a hidden no-op sub-action on the reusable env, which triggers GetOrCreateEnvironment on the backend and spins up min_replica_count workers. The heavy task — invoked later in the driver — lands on the warm pool instead of paying the cold start.

• await env.prewarm() blocks until the noo HEALTHY on return) — standard Python async semantics.
• asyncio.create_task(env.prewarm()) is the pool warms concurrently with whatever async work the driver does next.
• env.prewarm_sync() blocks until completiions.

The synthesized no-op is a real task registen time (__post_init__ calls _register_prewarm_task when reusable is not None). It shares the env's image, ReusePolicy, env_ve so its actor version hash matches the env's real tasks — without this match, the warm-up would land on a different pool a PrewarmTaskResolver materializes the task on the worker without touching user code, so the no-op is loadable even when thee bundle that doesn't define it explicitly.

Per-env naming (<env>.prewarm_<env>, prefix form) makes prewarm sub-actions sort together in the UI. prewarm() logs thfired so users can see the warm window they're operating in.

No-op cases (all warn and return):

• prewarm() on a non-reusable env
• Running in local execution mode
• Called outside a task / run context

Example Executions

Without Prewarm: https://union-internal.hosted.unionai.cloud/v2/domain/development/project/flytesnacks/runs/ux2bz5jljgkhlrxqt9st

With Prewarm: https://union-internal.hosted.unionai.cloud/v2/domain/development/project/flytesnacks/runs/uff6qnnx59wjzvx5f6kz

[kumare3]

looks cool, but you need to have one await before it really triggers sometime