fs: preserve readFile() errors for numeric file descriptors

[jibin7jose]

Summary

fs.readFile() with a numeric file descriptor and the new buffer option could lose errors on the user-fd path.

Root cause

In lib/internal/fs/read/context.js, ReadFileContext.close(err) did not forward err when isUserFd was true. As a result, failures that should have reached the callback, including an undersized caller-provided buffer, could be treated as successful completion.

Fix

Forward the close error for user-supplied file descriptors so the final callback receives the actual failure instead of silently succeeding.

Tests

Add regression coverage for the numeric file descriptor path with:

• a preallocated buffer
• a function-returned buffer
• a buffer that is too small

This verifies the error is preserved for the callback-based API.

Fixes: #64367

[jibin7jose]

Hi @joyeecheung, could you please take a look at this PR when you have a chance? It fixes an issue where fs.readFile() with a numeric file descriptor and the new buffer option could lose errors on the user-FD path. I've added regression tests covering the affected scenarios. Any feedback would be greatly appreciated. Thank you!

[joyeecheung]

I am not very familiar with this code any more, but it seems straightforward enough and from a glance it makes sense to me. Would be better if @addaleax can take a look since it was introduced in #35487

[joyeecheung]

Suggested change

	fs.readFile(fd, options, (err, data) => {
	if (err) reject(err);
	else resolve(data);
	});
	fs.readFile(fd, options, common.mustCall((err, data) => {
	if (err) reject(err);
	else resolve(data);
	}));

[jibin7jose]

Done, I've updated the code to wrap the callback with common.mustCall() as suggested. Thanks!