Fix MetaDrive simulator on macOS (spawn context, lazy imports, timestamp fix)

[mirathfan]

Description

Fixes #33207

Fix MetaDrive simulator support on macOS by addressing process startup and timing issues in the sim path.

Root causes:

• manager.py used unblock_stdout() / forkpty, which is unsafe in this macOS simulation path startup once Cocoa/Panda3D paths are involved
• MetaDrive subprocess startup used the default multiprocessing behavior, which is not safe enough for this macOS path
• Panda3D / MetaDrive imports happened too early in the parent process
• Sim camera EOF timestamps were synthetic instead of monotonic, which caused downstream freshness issues
• Movement detection during the macOS test run was too strict and could incorrectly report vehicle_not_moving

Fixes:

• Skip unblock_stdout() on Darwin when SIMULATION=1
• Lazy-load Panda3D / MetaDrive in the subprocess path
• Use multiprocessing "spawn" context on macOS for MetaDrive world to avoid fork-related instability with Cocoa/Panda3D
• Use time.monotonic() for sim camera EOF timestamps
• Lower movement threshold only for Darwin test runs to avoid false "vehicle_not_moving" detection due to smaller per-step motion

Verification

Ran locally on macOS:

python -m pytest -vv -s -n0 tools/sim/tests/test_metadrive_bridge.py

Result:

1 passed, 1 skipped

[github-actions]

Process replay diff report

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✅ 0 changed, 66 passed, 0 errors