Add Atlas Cloud provider support

[binyangzhu000-sudo]

Summary

• add an Atlas Cloud Media API image generator and register it with the existing image generator factory
• wire Atlas Cloud through image generation and config connection testing without changing the root README
• add Atlas Cloud image/text provider examples plus focused tests for the generator, config testing, and image service parameters

Validation

• uv run pytest tests/history_and_image_service_test.py tests/errors_test.py tests/atlascloud_image_test.py -q
• uv run python -m compileall backend
• git diff --check
• live Atlas model catalog check confirmed bytedance/seedream-v5.0-lite and qwen/qwen3.5-flash

Notes

• No root README changes.
• No sponsor, logo, credits, or partner promotion changes.