numfmt: handle output write errors

[cyphercodes]

Summary

• Propagate numfmt output write failures instead of unwrapping them in formatting writers.
• Report stdout write failures as write error: ... I/O errors with status 1.
• Add a Linux/Android /dev/full regression test to ensure this does not panic.

Tests

• cargo test -p uu_numfmt
• cargo test --no-default-features --features numfmt test_numfmt
• cargo clippy -p uu_numfmt -- -D warnings
• cargo fmt --all -- --check
• git diff --check

Fixes #12551

[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/date/date-locale-hour (passes in this run but fails in the 'main' branch)

[sylvestre]

Note : the section is comment 0 "Tests " isn't useful :)