fix(fold): use buffered read to prevent hang on infinite streams like /dev/zero

[SaathwikDasari]

Description

Fixes #9845

The previous implementation of fold attempted to read input line-by-line using read_until. When processing infinite streams without newlines (such as /dev/zero), this caused the internal buffer to grow indefinitely, leading to a hang or OOM (Out of Memory) crash.

Changes

• Refactored fold_file to use a fixed-size stack buffer (8KB) and Read::read instead of BufRead::read_until.
• Added logic to process and flush the buffer in chunks, allowing infinite streams to be processed continuously without exhausting memory.
• Added a regression test using /dev/null (guarded by #[cfg(unix)] to avoid Windows CI failures).

Verification

• Verified locally that fold /dev/zero no longer hangs and streams output correctly.
• Verified cargo test --package uu_fold passes.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/fold/fold-characters. tests/fold/fold-characters is passing on 'main'. Maybe you have to rebase?

[codspeed-hq]

CodSpeed Performance Report

Merging #9966 will degrade performance by 6.42%

_{Comparing SaathwikDasari:fix-fold-dev-zero (9a67924) with main (f60072d)}

Summary

❌ 1 regression
✅ 129 untouched
⏩ 30 skipped¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Efficiency
❌	fold_many_lines[100000]	80.4 ms	85.9 ms	-6.42%

Footnotes

1. 30 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/fold/fold-characters. tests/fold/fold-characters is passing on 'main'. Maybe you have to rebase?

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/fold/fold-characters. tests/fold/fold-characters is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/sort/sort-stale-thread-mem. tests/sort/sort-stale-thread-mem is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)

[ChrisDryden]

This has the same issue as the following PR: #10396 where multi byte characters that are at the end of a buffer are getting split into multiple characters.