Fix delayed observable sampling timing

[zjn20030811]

What this does

• fixes delayed observables sampling on the first simulator tick instead of at their scheduled offset
• keeps forced refreshes outside the periodic sampling clock, so querying a freshly set simulation state does not consume a scheduled sample or redraw its delay
• clears the sampled flag on reset
• adds focused CPU regressions for zero-delay, delayed, forced, and reset behavior

This restores the intended sensor timing used by delayed observations in sim-to-real experiments.

How it was tested

• python -m pytest tests/test_observables.py -q (4 passed)
• python -m black --check robosuite/utils/observables.py tests/test_observables.py
• python -m isort --check-only robosuite/utils/observables.py tests/test_observables.py

The regressions verify that a 100 ms observable with 30 ms delay samples at 70 ms, that a forced refresh preserves the current phase and sampled delay, and that reset starts a fresh sampling period.

How to checkout & try?

python -m pytest tests/test_observables.py -q