timeout: Improve error handling with explicit expect messages

[naoNao89]

Replace generic .unwrap() calls with .expect() containing descriptive messages that document critical assumptions about signal availability and argument validation.

• SIGTERM: .expect("TERM signal should always be valid")
• SIGKILL: .expect("KILL signal should always be valid")
• SIGCONT: .expect("CONT signal should always be valid")

These are POSIX.1 standard signals guaranteed on all Unix systems. See GNU manual - Signal specifications.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/overlay-headers (fails in this run but passes in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)

[codspeed-hq]

CodSpeed Performance Report

Merging #9435 will improve performance by 3.53%

_{Comparing naoNao89:timeout-error-handling (22b63ee) with main (666c6df)}

Summary

⚡ 1 improvement
✅ 138 untouched
⏩ 37 skipped¹

Benchmarks breakdown

	Benchmark	BASE	HEAD	Efficiency
⚡	tsort_input_parsing_heavy[5000]	73 ms	70.5 ms	+3.53%

Footnotes

1. 37 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. ↩