test(agent): add deterministic mock agent fixture

[Staaaaaaaaar]

Summary

• add a CPU-only rh_mock_agent fixture implementing Agent status, idempotent reset, PointNav task, Episode state, simulation clock, and cmd_vel contracts
• gate scripted commands on the active reset identity, one valid immutable task snapshot, and an accepted RUNNING state
• drive configurable command segments strictly from simulation time and clear task, command, and script state on each new Episode
• add controlled readiness, reset failure, heartbeat suppression, ERROR, and process-crash injection
• add deterministic unit and ROS black-box coverage, isolated from parallel fixtures with a dedicated DDS domain

Safety and scope

• reset and every non-RUNNING transition publish zero velocity
• mismatched Episodes, stale state sequences, invalid tasks, and conflicting task snapshots cannot open the command gate
• duplicate reset requests replay the original result without restarting the script
• this remains a protocol fixture: no planner, goal follower, keyboard input, learned policy, or platform adapter is introduced

Validation

• make dev-check
• 9 ROS packages built
• 159 tests passed, 0 errors, 0 failures, 0 skipped
• Mock Agent black-box suite completed in about 1.4 seconds