Fix ImageMagick metadata size parsing

[rissrice2105-agent]

Summary

• parse ImageMagick %b/%[b] size values with unit suffixes into bytes
• use the parser for image metadata format.size
• add unit tests for byte, KB, MB, and MiB values plus invalid inputs

Testing

• pnpm exec mocha test/utils.test.js

Note: pnpm test test/utils.test.js runs the entire test glob in this repo; the wider suite fails locally because ffmpeg/ImageMagick binaries are not available on this Windows PATH.