fix: validate fileSize is integer to prevent limit bypass

[abhu85]

Summary

Validate that fileSize limit is an integer to prevent silent limit bypass with floating-point values.

Problem

When limits.fileSize is a floating-point number (e.g., 1024.1), busboy does not enforce the limit correctly, allowing files larger than intended to be uploaded.

Solution

Add input validation in the Multer constructor to throw a TypeError if fileSize is not an integer, matching the behavior in multer v3. The check uses Number.isInteger() and only applies when fileSize is explicitly provided (non-null).

Test Plan

• fileSize: 1024.5 throws TypeError
• fileSize: 1024 works normally
• Existing tests pass (74/74)

Fixes #1132