Feat/upgrade echarts v6 (#20)

* feat: upgrade to ECharts 6.0.0

- Upgrade ECharts from ^5.6.0 to ^6.0.0
- All 54 tests pass successfully with ECharts 6
- Maintain full backward compatibility

* test(echarts): 添加图表快照测试及图像对比功能

添加了针对各种ECharts图表类型的快照测试，包括柱状图、折线图、饼图等15种图表类型
新增toImageEqual工具函数用于图像对比，支持设置最大误差阈值
添加了测试所需的pngjs和pixelmatch依赖

* refactor: split consolidated test file into individual chart type tests

- Split large all-charts.spec.ts into separate test files for each chart type
- Create individual test files: bar, line, pie, scatter, radar, gauge, funnel, heatmap, sunburst, treemap, tree, graph, sankey, boxplot, candlestick, parallel, echarts
- Each test file uses real chart tools from src/ directory
- Move shared test setup to __tests__/utils/matcher.ts
- Update extractImageBuffer to handle MCP tool response format correctly with proper type safety
- Generate comprehensive image snapshots for visual regression testing
- All 90 tests now pass with proper source code integration

Addresses code review feedback to improve test organization and maintainability.

* feat: unify legend position to bottom and fix ECharts 6.0 compatibility issues

- Unified all chart legends to bottom position with horizontal layout and center alignment
- Fixed legend configurations for line, bar, funnel, graph, candlestick, and gauge charts
- Updated test files from .test.ts to .spec.ts format and refactored image comparison logic
- Fixed ECharts 6.0 compatibility issues:
  * Candlestick chart grid layout for volume display
  * Multiple gauge chart layout to prevent overlapping
  * Series naming for proper legend matching

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Co-authored-by: liyongzhi02 <liyongzhi02@58.com>

Author: Susuperli