MAINT: consolidate duplicate image URL download handling

[Utkarsh Bahuguna (u7k4rs6)]

Summary

Consolidates the duplicated async image URL download implementation used by image converters.

Changes

• Added a private shared _download_image_from_url_async() helper.
• Retained _read_image_from_url_async() on both converter classes as protected extension points.
• Preserved dynamic subclass override behavior.
• Preserved existing aiohttp error handling, exception chaining, cancellation propagation, and raw byte behavior.
• Added focused tests for download behavior, error handling, cancellation, delegation, and subclass compatibility.

Compatibility

No public API or inheritance changes.

Existing callers and subclasses that override or patch _read_image_from_url_async() continue to use the same protected hook.

Validation

• 85 focused converter tests passed
• Ruff check passed
• Ruff format check passed
• Ty checks for changed converter/helper files passed
• git diff --check passed

Fixes #2417