[MRV2] Add routed-expert capture support on top of #44214

[aoshen02]

Summary

This stacked follow-up adds Model Runner V2 routed-expert capture support on top of xuanyu/enable-fi-monolithic-moe-rr from vllm-project#44214.

Before this change, Model Runner V2 only bound BaseRouter capture callbacks. Monolithic MoE kernels perform routing inside the fused expert kernel, so supported monolithic kernels from vllm-project#44214 never received the routed-expert capture callback and R3 results could be missing or all zero.

This change:

• keeps the existing modular BaseRouter capture path;
• discovers the monolithic fused_experts implementation through the V2 MoE quantization path;
• binds the replay callback only when supports_routing_replay_capture() is true;
• adds V2 binding coverage and keeps unsupported monolithic kernels unbound.

This PR is intentionally stacked on both vllm-project#38163 (Model Runner V2 expert-index capture) and vllm-project#44214 (FlashInfer monolithic replay support). The requested base branch is xuanyu/enable-fi-monolithic-moe-rr in xuanyu-mistral/vllm.

Validation

• git diff --check passes.
• The existing V2 support validation in [MRV2] Capture routed experts from supported monolithic MoE kernels vllm-project/vllm#49184 reports 2 tests passed and 4 deselected in the vllm/vllm-openai:nightly container.
• The related end-to-end [RL Infra][FlashInfer] Enable router replay output from FlashInfer monolithic MoE kernel vllm-project/vllm#44214 validation confirmed that an unsupported monolithic backend fails fast when R3 is enabled.

AI assistance was used. A human submitter should review every changed line and verify the stacked dependencies before merging.