Add coverage-focused tests for disk storage and middleware internals

[ashish3011]

• Added targeted test cases for the disk storage flow and multipart middleware internals to exercise previously uncovered branches.
• This improves confidence in the upload lifecycle around destination handling, error propagation, cleanup behavior, and field-name validation.
• The goal is to raise coverage for the affected internal modules and reduce the chance of regressions in core upload behavior.

Those two files were touched as a small implementation cleanup while I was adding the coverage tests.

lib/file-appender.js: I changed the placeholder-removal logic from the old bitwise-style check to a plain [idx >= 0] check. It is functionally equivalent, but it is clearer and easier to reason about.
lib/make-middleware.js: I changed the pending-file cleanup from [indexOf] + [splice] to a [filter]-based removal. That makes the removal step more robust when multiple completion paths are happening and avoids mutating the array in a less obvious way.