Testing foundation: add unit tests for models + core controllers (initial suite)

[syed-reza98]

Why

Docs report 0% test coverage; this blocks safe iteration.

Scope (initial)

• Model JSON roundtrip tests
• Controller state transition tests for critical flows

Tasks

• Add test/ structure (models/controllers)
• Add baseline tests:
  • PetModel, UserModel, PostModel, OrderModel JSON parse/serialize
  • AuthNotifier login/register error paths (mock repo)
  • CartController persistence (after cart persistence issue is implemented)
• Add CI workflow to run flutter test

Acceptance criteria

• flutter test runs in CI
• Core models covered with JSON tests
• At least one controller covered with state transition test

References

• docs/01_CODEBASE_ARCHITECTURE_REVIEW.md: Test Coverage 0/10
• docs/02_FEATURES_REQUIREMENTS_BEST_PRACTICES.md: Testing Strategy + CI blueprint

[syed-reza98]

Closed as completed via PR #71.