[2/5][BREAKING][rollout] feat: add the polling RolloutClient

[mathewjhan]

What

Adds osmosis_ai.rollout.client.RolloutClient. run_rollout() waits for a terminal result; await run_rollout_async() waits for admission and returns the polling task. Admission retries HTTP 429 with bounded Retry-After values and an optional admission timeout.

The server now creates the polling lease and includes it in the 202 response. The client sends that lease on result requests and exposes cancel_rollout(). Removes HttpRolloutDriver, RolloutRunRequest, and RolloutOutcome.

Why

Provides a shared client for the long-poll protocol introduced by #345 so consumers do not each implement admission and result polling.

Migration and dependencies

Breaking removal of the old driver API; use RolloutClient and RolloutResultResponse. Based on #345; #347 migrates local eval and #348 migrates the lifecycle benchmark. Stack: #345 → #346 → #347 → #348 → #349.

TrainGate still needs its own consumer migration; its existing callback/status clients cannot use this protocol unchanged.

How to Test

Run uv run pytest tests/unit/rollout/client tests/unit/rollout/test_server_admission.py, then test cancellation during an HTTP request.

Current CI is not green. Review reproduced a cancellation hang when cancellation reuses the just-cancelled request's HTTP client; a test-only comparison using a separate client passed. Public client attribute annotations also need to satisfy pyright --verifytypes.