[Quantization] Fix Humming MXFP4 activation selection

[HDCharles]

PR vllm-project#53880 selects Humming before considering compressed-tensors activation metadata, while the MXFP4 oracle treats Humming as BF16-only when an explicit activation override is present. This makes checkpoint-declared block FP8 and an equivalent quantization_config.moe.activation follow inconsistent paths.

This change treats Humming as a flexible-activation backend, forwards the resolved activation key to Humming's existing compatibility validation, and converts the checkpoint input schema to a QuantKey before oracle selection. Matching checkpoint/config declarations now agree, conflicting declarations use the oracle's existing conflict error, and the resolved schema is reused during Humming conversion.

This is a focused follow-up to vllm-project#53880 rather than a duplicate vLLM PR. It targets the source branch so its commit can be incorporated into that PR.

Tests:

• ruff check vllm/model_executor/layers/fused_moe/oracle/mxfp4.py vllm/model_executor/layers/quantization/compressed_tensors/compressed_tensors_moe/compressed_tensors_moe_w4a4_mxfp4.py tests/quantization/test_humming_mxfp4_block_fp8.py — passed
• python -m py_compile for the three changed Python files — passed through uv run
• git diff --check — passed
• pytest -q tests/quantization/test_humming_mxfp4_block_fp8.py — could not start because the available environment is missing cbor2

No model evaluation was run. The change affects backend/config selection and should be included in vllm-project#53880's model evaluation before merge.

AI assistance was used to review, implement, and test this change. The human submitter reviewed the resulting diff and is responsible for the contribution.

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[HDCharles]

Closing this stacked PR; the branch will instead be prepared for an upstream submission if the existing vllm-project/vllm PR is closed or superseded.