fix: allow diskStorage without options and handle string destination in template

[vaibhavmashal]

Summary

• Safely default \opts = opts || {}\ in \DiskStorage\ constructor so that calling \multer.diskStorage()\ without arguments does not throw a TypeError when accessing \opts.filename.
• Update the custom storage template in \StorageEngine.md\ to safely handle optional \opts\ and support \destination\ passed as either a \string\ or a function (resolves Potential Issue with destination Parameter Type in Multer Custom Storage #1278).
• Add unit test in \ est/disk-storage.js\ verifying \multer.diskStorage()\ with omitted options uses default destination and filename.

Verification

• 
  pm run lint\ passed.
• 
  pm test\ passed (133 passing).

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[kilisamemarisaaa]

Could we remove req.file.path before completing the new omitted-options test, and ideally clean up the adjacent default-destination test as well? I ran the new test alone on Windows while snapshotting os.tmpdir(): it passed, but left one new 1,778-byte file whose SHA-256 matched test/files/small0.dat. The full suite leaves two such files because successful uploads do not call _removeFile and the current afterEach only removes uploadDir.

[vaibhavmashal]

Thanks for the thorough catch and review @kilisamemarisaaa!

I've updated both the new omitted-options test and the adjacent default-destination test in \ est/disk-storage.js\ to call \s.unlink(req.file.path, done), ensuring any temporary files written to \os.tmpdir()\ are cleaned up before test completion.

Pushed in commit 4ebb5af — all 133 tests continue to pass cleanly.

[kilisamemarisaaa]

Reviewed head 4ebb5af. The omitted-options DiskStorage regression is covered, both successful temp-directory tests now unlink req.file.path, and the custom storage template handles omitted opts plus string destinations. I ran npm test (133 passing, 2 pending) and npm run lint locally on Windows. No blocking findings.

[vaibhavmashal]

Thank you for the thorough review and verification, @kilisamemarisaaa! Glad everything is clean and passing.

[vaibhavmashal]

Just to note regarding the 2 pending tests on Windows: those are the Linux-specific file descriptor tests in \ est/abort-fd-leak.js\ and \ est/abort-unlink-after-close.js\ which check /proc/self/fd\ and gracefully \skip\ on non-Linux platforms. On Linux (and CI), all 135 tests execute and pass cleanly.