Add ViewModel lifetime widget tests + run tests in publish workflow

[okmanideep]

Summary

• Add focused widget tests covering ViewModelScope/ViewModel lifetime rules (retention across rebuilds, disposal on unmount, keyed instances, remount behavior, nested scopes, per-route scopes).
• Run flutter test in publish workflow to prevent publishing on failing tests.

Rationale

• Ensure lifecycle invariants are enforced with fast, deterministic tests.
• Reduce release risk by failing CI before flutter pub publish.

Notes

• Updated example/pubspec.lock after pub get.
• No runtime code changes.

[jillellamudisurya]

LGTM Bro!! 👏