[vLLM][MoE] Avoid top-k boundary ties causing test failures

[sarah12121212]

Summary

test_fused_moe generates random BF16 routing scores for top-k selection. Different routing implementations may resolve scores tied at the selection boundary using different expert IDs, causing numerical failures unrelated to the correctness of the MoE kernels.

This PR:

• Detects if the kth and k+1th routing scores are equal.
• Rerolls rows containing a boundary tie.
• Fails if a tie-free input cannot be generated after 10 attempts.
• Unskips 30 affected xe2 cases and 28 corresponding default cases.

Validation

Full vLLM MoE suite on B580 (Tested locally):

926 passed
2,061 skipped
0 failed

Full vLLM MoE suite on max 1100 (Tested locally):

Blocked by #7462 (it needs to be merged first)