fix: resize ADB screenshots to model limit

[binichallein]

Summary

• Fixes 输入图片大小限制 #381 by resizing ADB screenshots before base64 encoding when either dimension exceeds the 2048 model input limit.
• Applies the same limit to fallback black screenshots so fallback images cannot trigger the same input-size error.
• Adds regression tests for tall screenshots and fallback screenshot dimensions.

Test Plan

• python -m pytest
• pre-commit run --files phone_agent/adb/screenshot.py tests/test_screenshot_resize.py