[Bugfix] Fix DFlash prefix cache corruption due to missing lookahead block (#4449)

Signed-off-by: Nick Cao <ncao@redhat.com>

vllm_omni/worker/gpu_ar_model_runner.py

@@ -1052,10 +1052,7 @@ class GPUARModelRunner(OmniGPUModelRunner, OmniConnectorModelRunnerMixin):
         spec_config = self.speculative_config
         propose_drafts_after_bookkeeping = False
         if spec_config is not None:
-            input_fits_in_drafter = spec_decode_common_attn_metadata is not None and (
-                spec_decode_common_attn_metadata.max_seq_len + self.num_spec_tokens
-                <= self.effective_drafter_max_model_len
-            )
+            input_fits_in_drafter = self._input_fits_in_drafter(spec_decode_common_attn_metadata)
             use_gpu_toks = (
                 spec_config.use_eagle() or spec_config.uses_draft_model() or spec_config.uses_extract_hidden_states()
             ) and not spec_config.disable_padded_drafter_batch