fix: support List[<MCP types>] return values for mcp_tool

[hallvictoria]

Fixes an issue where functions with List[<MCP types>] return values were not returned correctly.

Example function:

@app.mcp_tool()
@app.mcp_tool_property(arg_name="label", description="The label text for the badge.", is_required=True)
@app.mcp_tool_property(arg_name="value", description="The value text for the badge.", is_required=True)
@app.mcp_tool_property(arg_name="color", description="The hex color for the value section (e.g., '#4CAF50').", is_required=False)
def generate_badge(label: str, value: str, color: str = "#4CAF50") -> List[ContentBlock]:
    """Demonstrates returning multiple content blocks (List[ContentBlock]). Generates an SVG status badge and returns it alongside a text description."""
    logging.info(f"Generating badge: {label} | {value}")
    
    label_width = len(label) * 7 + 12
    value_width = len(value) * 7 + 12
    total_width = label_width + value_width
    
    svg = f"""<svg xmlns="http://www.w3.org/2000/svg" width="{total_width}" height="20">
  <rect width="{label_width}" height="20" fill="#555"/>
  <rect x="{label_width}" width="{value_width}" height="20" fill="{color}"/>
  <text x="{label_width // 2}" y="14" fill="#fff" text-anchor="middle"
        font-family="Verdana,sans-serif" font-size="11">{label}</text>
  <text x="{label_width + value_width // 2}" y="14" fill="#fff" text-anchor="middle"
        font-family="Verdana,sans-serif" font-size="11">{value}</text>
</svg>"""
    
    return [
        TextContent(type="text", text=f"Badge: {label} — {value}"),
        ImageContent(
            type="image",
            data=base64.b64encode(svg.encode('utf-8')).decode('utf-8'),
            mimeType="image/svg+xml"
        )
    ]