fix: allow files at exact fileSize limit

[raashish1601]

What changed

• Increase configured ileSize limit passed to Busboy by 1 byte only when limits.fileSize is explicitly set.
• This avoids treating an upload exactly equal to limits.fileSize as over-size while preserving the current error for larger uploads.
• Add regression test for exact-boundary acceptance in est/error-handling.js.

Fixes #1348 (off-by-one file size limit behavior).