Add Unit Tests to Dapr Agents

[bibryam]

Context
Dapr Agents is approaching a stable release, and maintaining reliability and code quality is critical. Adding unit tests will help catch regressions early, ensure core logic is testable, and support future contributions with confidence.

Acceptance Criteria

• Add unit test coverage to core components
• Ensure tests are runnable locally via standard tooling
• Add test execution to CI pipeline.

[sicoyle]

closing as good enough for now since we've made a lot of progress on this front :)