improvement: extract and deliver chart PNGs from code_run output

[hjpinheiro]

Summary

When Python code generates charts (matplotlib, seaborn, plotly, etc.), the Daytona SDK captures them as base64 PNG images in response.artifacts.charts. These were previously discarded.

This PR extracts the chart PNGs and delivers them as blob messages so users see rendered charts directly in the Dify chat.

Changes

Chart PNG delivery

# Extract PNG from chart elements and deliver as image
for chart in response.artifacts.charts:
    for element in chart.elements:
        if element.png:
            png_bytes = base64.b64decode(element.png)
            yield self.create_blob_message(blob=png_bytes, meta={"mime_type": "image/png"})

Chart metadata in JSON response

{
    "exit_code": 0,
    "output": "...",
    "charts_count": 2,
    "charts": [{"type": "bar", "title": "Sales by Quarter"}, ...]
}

Text output

Added create_text_message(response.result) so the agent sees code output in chat (previously only JSON was returned).

SDK structure

The chart data path (verified against daytona==0.187.0):

ExecuteResponse.artifacts: ExecutionArtifacts
  └── .charts: list[Chart]
       └── .elements: list[GeneratedChartElement]
            ├── .png: str (base64-encoded PNG)
            ├── .type: str (chart type)
            └── .title: str (chart title)

All field access uses defensive getattr with None defaults.

Testing

• py_compile passes