[Bugfix] Add missing draft_attn_metadatas parameter to fix MTP test (#6232)

### What this PR does / why we need it?

Fix the MTP test failure caused by accessing non-existent attribute
`forward_context.draft_attn_metadatas`.

**Root cause:**
In `AscendAttentionBackendImpl.update_graph_params`, the code
incorrectly accessed `forward_context.draft_attn_metadatas`, but
`ForwardContext` class doesn't have this attribute. The original code
passed this value via function parameter.

**Fix:**
Add `draft_attn_metadatas` parameter to the entire call chain:
- `update_full_graph_params` function in `acl_graph.py`
- All `update_graph_params` methods in attention backends
- Pass the parameter correctly in `eagle_proposer.py`

Also applied Gemini's suggestion to make `vllm_config=None` in
`AscendAttentionCPImpl.update_graph_params` for API consistency.

Related to item 9 in #5463

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

This fixes the CI test failure:

`test_deepseek_mtp_correctness[True-FULL_DECODE_ONLY-2-wemaster/deepseek_mtp_main_random_bf16]`

Signed-off-by: lico67373 <918688502@qq.com>

vllm_ascend/spec_decode/eagle_proposer.py

@@ -1184,7 +1184,8 @@ class EagleProposer(VllmEagleProposer):
     def _update_full_graph_params(self, forward_context, num_tokens, draft_attn_metadatas=None):
         update_full_graph_params(
             self.runner.attn_backend, self.update_stream, forward_context, num_tokens,
-            self.vllm_config, self.vllm_config.speculative_config)
+            self.vllm_config, self.vllm_config.speculative_config,
+            draft_attn_metadatas=draft_attn_metadatas)
 
     # padding tensor into desired size
     def _pad_tensor(self, tensor, pad_size):