python: guard _simulate bindings behind MUJOCO_BUILD_SIMULATE

[arushsinghal]

What this PR does

Guards the Python _simulate extension behind MUJOCO_BUILD_SIMULATE
and the presence of mujoco::libmujoco_simulate.

This prevents Python wheel builds from failing when MuJoCo is built
without the simulate component (e.g. headless or minimal builds).

Why this is needed

The Python bindings currently attempt to unconditionally build
_simulate, which causes CMake configuration errors if
libmujoco_simulate is not available.

This change aligns the Python build behavior with core MuJoCo
build options.

Testing

• Built MuJoCo without MUJOCO_BUILD_SIMULATE
• Verified Python bindings no longer attempt to build _simulate
• No behavior change when MUJOCO_BUILD_SIMULATE=ON

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[arushsinghal]

This change guards the Python _simulate extension behind
MUJOCO_BUILD_SIMULATE and avoids referencing libmujoco_simulate
when the target is not present.

Tested locally by toggling MUJOCO_BUILD_SIMULATE=ON/OFF and verifying
wheel build behavior.