[Fix] Forward SWA prealloc reclaim through the DSV4 HiSparse allocator (#40354)

Co-authored-by: Mohammad Angkad <mohammad.angkad@radixark.ai>

python/sglang/srt/mem_cache/allocator/hisparse.py

@@ -376,6 +376,14 @@ class DeepSeekV4HiSparseTokenToKVPoolAllocator(BaseTokenToKVPoolAllocator):
     def swa_available_size(self):
         return self.logical_attn_allocator.swa_available_size()
 
+    def reclaim_for_prealloc(
+        self, tree_cache, full_tokens: int, swa_tokens: int
+    ) -> str | None:
+        # C4 needs no reclaim here: full_available_size prices it into the budget.
+        return self.logical_attn_allocator.reclaim_for_prealloc(
+            tree_cache, full_tokens, swa_tokens
+        )
+
     def free_swa(self, free_indices: torch.Tensor):
         self.logical_attn_allocator.free_swa(free_indices)