Owen/extensions tests p1

[OwenMcDonnell]

Description

Add flet-charts example tests and images.

Summary by Sourcery

Add screenshot-based integration tests and documentation images for flet-charts examples and wire them into the docs build.

New Features:

• Introduce screenshot integration tests for line, bar, pie, candlestick, and Matplotlib chart examples using FletTestApp.

Enhancements:

• Add usage examples to chart class docstrings for bar, line, and candlestick charts.
• Update chart documentation pages to reference generated golden images and static PNGs instead of GIFs.
• Guard chart example scripts with main checks to enable import in tests.
• Register flet-charts integration test images in the MkDocs build so they are published with the docs.