Add Dependabot configuration for dependency updates

[8007305]

Adds Dependabot version updates for the two uv-managed Python projects plus GitHub Actions.

Two corrections made from the original draft:

• package-ecosystem changed from pip to uv for the /executor and /code-interpreter entries, since dependencies are tracked via uv.lock, not a pip requirements file. Dependabot's pip ecosystem won't correctly resolve or update uv.lock.
• Removed a reviewers: ["onyx-dot-app/python-sandbox"] entry that referenced the repo name rather than a real GitHub team, which would have silently failed to assign a reviewer. Happy to add the correct team/username back in if there is one.