[Core][MoE] Add explicit expert-parallel topology

[LiuYinfeng01]

Summary

• add an experimental --expert-parallel-size independent of tensor parallel size
• construct fixed-TP-lane EP groups, e.g. TP4/DP2 gives [0,4], [1,5], [2,6], [3,7]
• preserve existing flattened EP behavior when the option is unset
• fail closed for PP/PCP, EPLB, elastic EP, non-AG/RS backends, and EP sizes other than DP

This is the topology foundation for experts that remain tensor-sharded while ownership is distributed across DP ranks. MoE execution support is stacked in #52100.

Test plan

• pytest -q tests/config/test_explicit_expert_parallel.py — 9 passed
• Ruff check and format check on changed files
• git diff --check

[claude]

Claude Code Review

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