Fix empty connector version selector

[rmorlok]

Summary

• Replace the empty change-version selector with an explanatory state when a connector has no eligible versions.
• Keep migration and rollback controls available only when an eligible target exists.
• Add regression coverage for a connector with just its current version.

Closes #807

Verification

• yarn workspace @authproxy/admin test ConnectionDetail.test.tsx
• yarn workspace @authproxy/admin typecheck
• yarn workspace @authproxy/admin lint
• ./scripts/preflight.sh

Screenshots

Before (reported in #807)

After