Add Unit Tests for FEO Scheduler

[MohamedRady-99]

What

The core FEO stack currently relies heavily on system-level integration tests (which spin up full OS child processes). To improve test isolation, execution speed, and edge-case coverage, we need dedicated unit tests for internal components—specifically the Scheduler.

Acceptance Criteria (DoD)

• Introduce a thread-safe MockConnector implementing the ConnectScheduler trait in src/feo/src/scheduler.rs to intercept and record signal transmissions without networking.
• Add a unit test verifying activity dependency resolution (step_ready_activities) using the mock connector.
• Add a unit test verifying that graceful shutdown only targets successfully started activities (shutdown_gracefully).
• Add a rust_test rule to src/feo/BUILD.bazel so Bazel can discover and run these inline tests.

How

No response