[BugFix] Fix Llama4 - Index Error When Single Request Near Max Context (#16209)

Signed-off-by: Lucas Wilkinson <lwilkinson@neuralmagic.com>

vllm/v1/attention/backends/flash_attn.py

@@ -264,7 +264,7 @@ def make_local_attention_virtual_batches(
         np.arange(pages_per_local_batch, dtype=np.int32),
         (virtual_batches, pages_per_local_batch)) \
             + np.expand_dims(block_starts, axis=1)
-    block_indices = block_indices.flatten()
+    block_indices = block_indices.flatten().clip(max=block_table.shape[1] - 1)
     batch_indices = np.repeat(np.arange(actual_batch_size, dtype=np.int32),
                               local_blocks * pages_per_local_batch)
     block_table_local = block_table[batch_indices, block_indices]\