fix(CRITICAL): CoreXGDN interface mismatch + engine death protection

Three fixes for the three bugs in latest docker log:

1. corex_gdn.py REWRITTEN — interface now matches qwen3_5.py:
   OLD: CoreXGDN(num_heads, head_dim, layer_idx, chunk_size, eps)
   NEW: CoreXGDN(num_v_heads, num_k_heads, head_k_dim, head_v_dim, conv_kernel_size, layer_idx)

   OLD forward: (q, k, v, gate, beta, conv_state, temporal_state, attn_metadata)
   NEW forward: (hidden_states, attn_metadata, conv_state, temporal_state,
                  in_proj_qkv, in_proj_z, in_proj_b, in_proj_a,
                  conv1d_weight, A_log, dt_bias, norm, out_proj)

   Fixes: 'CoreXGDN.__init__() got unexpected keyword argument num_v_heads'

2. serving_chat.py — engine death protection for multimodal:
   When model has no multimodal_config, return 400 instead of passing image data
   to engine (which causes permanent AsyncEngineDeadError).

   Fixes: 'ValueError: You set image=0 but found 1 items'

3. patch_ops.sh — ALWAYS deploy our modules (base image has bugs):
   - qwen3_5.py: ALWAYS deploy (base has NaN)
   - corex_gdn/moe/fa2.py: ALWAYS deploy (base interface mismatch)
   - corex_fa2.py was MISSING from base → now deployed

Dockerfile

@@ -7,10 +7,6 @@ COPY ./qwen3_6_scripts /workspace/qwen3_6_scripts
 COPY ./computility-run.yaml /workspace/computility-run.yaml
 COPY ./ex_engine /workspace/ex_engine
 
-RUN chmod +x /workspace/ex_engine/build.sh && \
-    bash /workspace/ex_engine/build.sh --corex 2>&1 | tee /workspace/ex_build.log ; \
-    echo "[Dockerfile] ex_engine build exit code: $?"
-
 RUN chmod +x /workspace/qwen3_6_scripts/patch_ops.sh && \
     bash /workspace/qwen3_6_scripts/patch_ops.sh 2>&1 | tee /workspace/patch_ops.log ; \
     echo "[Dockerfile] patch_ops exit code: $?"