fix(notebooks): fall back to SITE_URL for sandbox callback URLs in prod

[sinan-ku]

Problem

Kernel-lane SQLV2 notebook runs (python/duckdb) never complete outside local dev. The sandbox is handed its callback and data-plane base URL by _backend_base_url(), which falls back to http://host.docker.internal:8000 whenever SANDBOX_API_URL is unset. That's a dev-only Docker alias, unreachable from a remote sandbox, so the kernel executes the code but can never POST the result envelope back to /internal/notebooks/runs/<run_id>/result/ (or reach the data plane). The run row stays RUNNING forever and the node spins indefinitely, even for a trivial print(1+2).

Changes

_backend_base_url() now resolves in order:

1. SANDBOX_API_URL if set (e.g. an ngrok tunnel for driving a Modal sandbox from local dev)
2. http://host.docker.internal:8000 under DEBUG (the local kernel runs in Docker, where SITE_URL would be an unreachable localhost)
3. SITE_URL otherwise

This matches how the tasks product resolves its sandbox API URL (SANDBOX_API_URL or SITE_URL), with the extra dev branch because notebooks' local kernel lives in a Docker container.

How did you test this code?

Added TestSQLV2BackendBaseURL (parameterized SimpleTestCase, no DB) covering the three branches through build_callback_url. It catches the regression that shipped: a deployed environment without SANDBOX_API_URL handing the sandbox a dev-only unreachable URL - no existing test exercised these URL builders. Ran locally: 3 passed.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

Docs update

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Authored with Claude Code. Skills invoked: /writing-tests. Root cause was diagnosed in an earlier session by tracing prod run rows and service logs: stuck-running kernel runs with zero requests ever hitting the callback or data-plane endpoints, pointing at the unreachable fallback URL. Considered fixing this purely via deployment env (SANDBOX_API_URL on the worker fleet) but the code-side SITE_URL fallback works with no deploy coordination and mirrors the existing tasks sandbox behavior; the env override remains available on top.

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