Add gripper blocking option and enable default wait behavior

[ZidongChen25]

What changed

• bigym/action_modes.py

  • Added block_gripper_until_reached: bool (default True)
  • Added gripper_tolerance: float (default 0.05)
  • step() now enters internal stepping loop when either:
    • block_until_reached=True, or
    • block_gripper_until_reached=True
  • _is_target_state_reached() now optionally checks gripper convergence.

• bigym/robots/gripper.py

  • Added target tracking for the last commanded gripper control.
  • Added is_target_reached(tolerance) to determine whether gripper position is close enough to
    target.

Why

This makes gripper behavior closer to “command is discrete, motion is physical and progressive, API
returns after convergence”, which improves consistency for demo replay/rendering pipelines that assume
post-action gripper state.

Behavior impact

• JointPositionActionMode default behavior changes:
  • Gripper wait is now ON by default.
• This may increase per-step runtime in environments/scripts that do not override the new option.