fix(sdk): initialize sandbox annotations

[drew]

Summary

Fix the SDK test failures introduced when #1862 merged after the sandbox annotations protobuf changes. The SDK and its mock gateway now initialize the new annotation maps, restoring compilation on main.

Related Issue

Follow-up to #1862 and commit e6f319c.

Changes

• Initialize CreateSandboxRequest.annotations in the curated SDK client
• Initialize ObjectMeta.annotations in the mock gateway fixture
• Assert that curated sandbox creation sends an empty annotations map

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (not applicable)

Additional verification:

• cargo test -p openshell-sdk
• mise run test

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (not applicable; no behavior or architecture change)