[PCP][DCP] Declare FlashMLASparse MTP support at CP interleave > 1 (#56722)

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

vllm/v1/attention/backends/mla/flashmla_sparse.py

@@ -634,6 +634,7 @@ class FlashMLASparseMetadataBuilder(
 class FlashMLASparseImpl(SparseMLACommonImpl[FlashMLASparseMetadata]):
     can_return_lse_for_decode: bool = True
     supports_dcp: bool = True
+    supports_mtp_with_cp_non_trivial_interleave_size: bool = True
 
     @staticmethod
     def _compute_fp8_decode_padded_heads(num_heads: int) -> int: