Fork a sandbox with new environment variables

[devin-ai-integration]

Summary

Exposes the control plane's new SandboxForkRequest.envs (blaxel-ai/controlplane#5318, applied by blaxel-ai/executionplane#669) so a fork can be created with its own environment in one call, instead of forking then updating:

sandbox.fork("my-fork", envs=[Env(name="MODE", value="fork"), Env(name="FORK_ONLY", value="1")])

The list is an override on top of the source's environment: a name the source already carries takes the value given here, a name it does not is added, everything else is inherited.

client/models/sandbox_fork_request.py is regenerated from the controlplane branch (envs: Union[Unset, list["Env"]]); the async and sync SandboxInstance.fork() take envs and set it on the body only when given, so a fork without envs sends exactly the body it sends today.

The end-to-end check lives in the TypeScript SDK (blaxel-ai/sdk-typescript#438, tests/manual/fork_with_environment.ts); it needs the controlplane branch deployed, so it has not been run against dev yet.

Link to Devin session: https://app.devin.ai/sessions/6411382450434b99a696037879eb2b3a
Open in Devin Desktop: https://app.devin.ai/desktop/session/6411382450434b99a696037879eb2b3a?variant=devin
Requested by: @drappier-charles

[devin-ai-integration]

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