fix(sandbox): restrict provider file mode

[drew]

Summary

Provider-managed files are delivered through sealed, read-only memfds, but the descriptors report mode 777 by default. Set their mode to 600 so sandbox workloads inspecting file metadata see private permissions.

Related Issue

Closes #4091

Changes

• Set mode 600 on each provider memfd before serving it.
• Assert the mode in a unit test and from inside a Docker sandbox using the provider profile file path.

Testing

• Checks appropriate to the affected code and behavior pass (commit hook, including Rust format and lint)
• Unit tests added/updated: cargo test -p openshell-sandbox provider_files --lib (3 passed)
• E2E tests added/updated: Docker provider_files test (1 passed)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (not applicable)

[github-actions]

Label test:e2e applied for f68d7e7. Open the existing run and click Re-run all jobs to execute with the label set. The run will execute the standard E2E suite after building the required gateway, sandbox, and supervisor images once. The matching required CI gate status on this PR will flip green automatically once the run finishes.