fmt: propagate read errors instead of silently treating them as EOF

[gabrielhnf]

Fixes #10581

FileLines iterator was silently swallowing IO errors by returning None on any read failure, making them indistinguishable from end-of-file. This caused fmt to exit successfully with code 0 when reading files that produce IO errors, such as /proc/self/mem.

Changes:

• FileLines::Item is now Result<Line, std::io::Error>, propagating read errors to callers
• Added ParagraphStreamError enum to distinguish between unformatted passthrough lines and IO errors
• ParagraphStream::Item is now Result<Paragraph, ParagraphStreamError>
• process_file now returns an error on ParagraphStreamError::IoError instead of iterating over an empty stream
• Added a regression test that verifies $ fmt /proc/self/mem fails with exit code 1.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/cut/cut-huge-range is now passing!