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