Fix: code supports matplotlib (#13724)

### What problem does this PR solve?

Code as "final" node: 

![img_v3_02vs_aece4caf-8403-4939-9e68-9845a22c2cfg](https://github.com/user-attachments/assets/9d87b8df-da6b-401c-bf6d-8b807fe92c22)

Code as "mid" node:

![img_v3_02vv_f74f331f-d755-44ab-a18c-96fff8cbd34g](https://github.com/user-attachments/assets/c94ef3f9-2a6c-47cb-9d2b-19703d2752e4)


### Type of change

- [x] New Feature (non-breaking change which adds functionality)

agent/sandbox/sandbox_base_image/python/Dockerfile

@@ -2,12 +2,15 @@ FROM python:3.11-slim-bookworm
 
 COPY --from=ghcr.io/astral-sh/uv:0.7.5 /uv /uvx /bin/
 ENV UV_INDEX_URL=https://pypi.tuna.tsinghua.edu.cn/simple
+ENV MPLBACKEND=Agg
+ENV MPLCONFIGDIR=/tmp/matplotlib
 
 COPY requirements.txt .
 
 RUN grep -rl 'deb.debian.org' /etc/apt/ | xargs sed -i 's|http[s]*://deb.debian.org|https://mirrors.tuna.tsinghua.edu.cn|g' && \
     apt-get update && \
     apt-get install -y curl gcc && \
+    mkdir -p /tmp/matplotlib && \
     uv pip install --system -r requirements.txt
 
 WORKDIR /workspace

agent/sandbox/sandbox_base_image/python/requirements.txt

@@ -1,3 +1,4 @@
 numpy
 pandas
+matplotlib
 requests