Fix SonicMoE JIT lock warm-cache path

[A-nnonymous]

Summary

This PR fixes a multi-rank SonicMoE JIT lock stall when a prebuilt extension already exists.

Previously, all ranks entered an exclusive FileLock before attempting to import an existing .so. If one rank stalled while holding the lock during import/dlopen/build-related work, other ranks kept waiting even though the extension artifact was already available.

The fix:

• imports existing prebuilt extensions before acquiring the build lock;
• keeps FileLock only around the producer/build path;
• exits the lock before importing artifacts produced by another process;
• adds SONIC_MOE_JIT_LOCK_TIMEOUT with a default of 600 seconds.

Validation

• python -m py_compile sonicmoe/jit.py passed.
• git diff --check passed.
• The source file and PaddleFleet runtime symlink copy were verified identical locally.
• The fix was previously validated in xiaoyao's allgather runtime where the SonicMoE lock spam disappeared and execution progressed past the JIT lock point.

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