shuf: handle default RNG init failures gracefully

[jorgitin02]

Fixes #11152.

This avoids panicking when default RNG initialization fails (for example when getrandom returns EAGAIN under syscall fault injection).

Changes:

• Switch default shuf RNG init to fallible StdRng::try_from_os_rng.
• Convert RNG init failure into a normal user-facing error path.
• Add regression/safety tests for default/seed/file random-source paths.

[oech3]

Would you fallback instead of err? GNU's test suite decided to accept it.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/rm/isatty is no longer failing!

[codspeed-hq]

Merging this PR will degrade performance by 3.38%

❌ 1 regressed benchmark
✅ 293 untouched benchmarks
⏩ 42 skipped benchmarks¹

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

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	shuf_input_range[1000000]	84.2 ms	87.1 ms	-3.38%

---

_{Comparing jorgitin02:fix/11152-strace-e-inject-getrandom-error-eaga (4e0ee46) with main (ba94c69)}

Footnotes

1. 42 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. ↩

[jorgitin02]

Would you fallback instead of err? GNU's test suite decided to accept it.

This pr only touches shuf, so the cp_large_file[16] slowdown looks
unrelated. ill wait for the latest run and acknowledge if it stays the only regression.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/date/resolution (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/rm/isatty is no longer failing!

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/resolution (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/rm/isatty is no longer failing!

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/rm/isatty is no longer failing!

[oech3]

Please drop unrelated stdbuf commit.

[jorgitin02]

Please drop unrelated stdbuf commit.

done

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/rm/isatty is no longer failing!
Note: The gnu test tests/seq/seq-epipe is now being skipped but was previously passing.

[oech3]

Flakeyness of shuf_input_range will be removed by #11169 .

[jorgitin02]

Closing this version and preparing a fresh follow-up.