Provider-owned startup

[burtenshaw]

This PR lets EnvClient start providers that own their image/source, so that Daytona and Modal are consistent with HFSandbox and this improved DX:

with BrowserGymEnv(
    message_timeout_s=120.0,
    max_message_size_mb=100.0,
    provider=provider,
).sync() as browsergym_env:
    env.reset()

[bot-ci-comment]

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[burtenshaw]

Required by #841

[sergiopaniego]

thanks! some comments from review:

Two that have no line to anchor inline:

• Docs. docs/source/guides/runtime-providers.md still shows only the old manual flow (provider.start_container(image) + wait_for_ready + MyEnv(base_url=..., provider=...)). It should show the new MyEnv(provider=provider) form and document new_session() (including that it can fail at MAX_CONCURRENT_ENVS capacity and that its lifecycle is owned by the parent).
• Coordination with #841. Both PRs modify env_client.py's provider plumbing independently and are not stacked, so there is a conflict to reconcile. Merging #880 first and rebasing #841 on top is the cleanest order, since #841 already dropped its own new_session expecting this PR to provide it.

[burtenshaw]

Thanks, addressed the review in 0403170.

• Updated docs/source/guides/runtime-providers.md with the MyEnv(provider=provider) flow, parent-owned lifecycle, and new_session() lifecycle/capacity behavior.
• Added ACASandboxProvider constructor image/env_vars parity with Modal/Daytona, plus focused unit coverage.
• Hardened async/sync child teardown, added the clear provider-owned startup error, documented new_session(), dropped unused track, and filtered child constructor kwargs.

Agreed on #841: #880 should land first, then #841 can rebase on top and use the shared provider/session plumbing from this PR.

[sergiopaniego]

lgtm!