Shorten PID controller test simulation

[RussTedrake]

Summary

• Shorten the PID controller grading simulation from 2.0 s to 1.25 s.
• Keep the 1e-4 s plant time step, controller gains, and existing final-position assertion unchanged.

Verification

• solutions/notebooks/robot/test_notebooks_solutions_robot.py::test_04_pid_controller passed locally (1 passed). The local run used a temporary localhost Meshcat port override because port 7000 was already occupied, plus model packages prefetched to /tmp.
• Headless simulation experiment at the unchanged 1e-4 s time step produced a final-position error norm of 0.01833 at 1.25 s (assertion limit: 0.05).
• Pre-commit hooks passed.

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This change is