[None][test] Add coverage for MsaSparseGqaFmha.forward

[StanleySun639]

@coderabbitai summary

Description

Adds regression coverage for behavior introduced in 6af4e0be435a.

tests/unittest/_torch/attention/sparse/test_minimax_m3_msa_backend.py::test_msa_fmha_forward_falls_back_to_eager_plan_when_decode_plan_is_none verifies: MsaSparseGqaFmha.forward now uses a two-level fallback: if msa_decode_gqa_plan (or msa_decode_dense_plan) is None, it reads msa_eager_gqa_plan (or msa_eager_dense_plan) via getattr. Previously it used a ternary that could pass None to run_msa_paged_gqa.

Test Coverage

• Test case: unittest/_torch/attention/sparse/test_minimax_m3_msa_backend.py::test_msa_fmha_forward_falls_back_to_eager_plan_when_decode_plan_is_none
• Jenkins log: Jenkins #23 — SUCCESS

Gap Fixer Validation

• Patch review: The revised patch correctly addresses the prior feedback by using a thin subclass with a custom init and @Property override to bypass the read-only 'attn' property, and the test directly exercises MsaSparseGqaFmha.forward with meaningful assertions on the fallback behavior.
• Gap Fixer run: Job #45

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