Point the opencode example at openenv.core.sandbox

[sergiopaniego]

What does this PR do?

The shared sandbox layer (SandboxBackend / SandboxHandle / ExecResult, the E2B and Hugging Face backends, the interception proxy) is moving from opencode_env.sandbox to openenv.core.sandbox in huggingface/OpenEnv#1010 (it closes the pi_env → opencode_env peer dependency). This updates the opencode example's one sandbox import so it keeps working once that lands:

-from opencode_env.sandbox.base import ExecResult, SandboxHandle
+from openenv.core.sandbox import ExecResult, SandboxHandle

Warning

Draft — do not merge until huggingface/OpenEnv#1010 lands. openenv.core.sandbox does not exist until that PR merges, so this import would break against the current OpenEnv.

Depends on: huggingface/OpenEnv#1010

Fixes # (issue)

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline, Pull Request section?
• Was this discussed/approved via a GitHub issue? Please add a link to it if that's the case.
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

AI writing disclosure

• AI-assisted: some parts were suggested or improved by AI, but the PR was written and reviewed by a human.

Who can review?

@AmineDiro