Reject stdin reads while output is captured

[MatthewMckee4]

Summary

Reject Python stdin reads with an actionable error while output capture is active, and redirect file descriptor 0 to immediate EOF for child processes. Preserve real stdin under --no-capture. Closes #983.

Test Plan

Ran just test and uvx prek run -a.

[github-actions]

❌ Merging this PR may alter performance

Baseline: main (6c8b73a). Candidate: PR (a5dc0b3). Each benchmark compares median CLI wall time on one GitHub Actions runner, alternating install order. Runs warm the duration cache before measuring and include default per-test status output. Lower is better.

⚡ 0 improved benchmarks
❌ 3 regressed benchmarks
✅ 8 unchanged benchmarks

Warning

Benchmark regressions were detected. Review the wall-time changes before merging.

Performance Changes

	Mode	Benchmark	Base	Head	Change	Runs
❌	WallTime	itsdangerous	201.9 ms	211.6 ms	+4.8%	21
❌	WallTime	blinker	239.1 ms	268.3 ms	+12.2%	21
❌	WallTime	tomlkit	694.8 ms	725.7 ms	+4.5%	15

All benchmark scores

	Mode	Benchmark	Base	Head	Change	Runs
✅	WallTime	requests	354.2 ms	354.1 ms	-0.0%	21
✅	WallTime	h11	425.3 ms	424.1 ms	-0.3%	21
✅	WallTime	sniffio	278.9 ms	279.5 ms	+0.2%	21
❌	WallTime	itsdangerous	201.9 ms	211.6 ms	+4.8%	21
❌	WallTime	blinker	239.1 ms	268.3 ms	+12.2%	21
✅	WallTime	jinja	360.9 ms	361.1 ms	+0.0%	21
✅	WallTime	installer	397.1 ms	398.4 ms	+0.3%	21
❌	WallTime	tomlkit	694.8 ms	725.7 ms	+4.5%	15
✅	WallTime	outcome	313.4 ms	314.0 ms	+0.2%	21
✅	WallTime	pluggy	309.5 ms	309.6 ms	+0.0%	21
✅	WallTime	werkzeug	1.531 s	1.531 s	+0.0%	21

[github-actions]

✅ Merging this PR will not alter memory usage

All benchmark scores

	Mode	Benchmark	Base	Head	Change	Runs
✅	Memory	requests	58.4 MiB	58.5 MiB	+0.2%	21
✅	Memory	h11	53.0 MiB	53.0 MiB	-0.0%	21
✅	Memory	sniffio	47.0 MiB	47.0 MiB	-0.0%	21
✅	Memory	itsdangerous	44.1 MiB	44.1 MiB	+0.0%	21
✅	Memory	blinker	46.8 MiB	46.8 MiB	+0.0%	21
✅	Memory	jinja	54.0 MiB	54.1 MiB	+0.2%	21
✅	Memory	installer	48.4 MiB	48.5 MiB	+0.2%	21
✅	Memory	tomlkit	48.9 MiB	48.9 MiB	+0.0%	15
✅	Memory	outcome	48.1 MiB	48.1 MiB	+0.0%	21
✅	Memory	pluggy	42.9 MiB	42.9 MiB	+0.0%	21
✅	Memory	werkzeug	84.2 MiB	84.2 MiB	+0.0%	21

[github-actions]

✅ No diagnostic changes

Test result comparison

Project	Previous	New
requests	✅ 117 pass · 0 fail · 12 skip	✅ 117 pass · 0 fail · 12 skip
h11	✅ 78 pass · 0 fail · 0 skip	✅ 78 pass · 0 fail · 0 skip
sniffio	✅ 3 pass · 0 fail · 1 skip	✅ 3 pass · 0 fail · 1 skip
itsdangerous	✅ 10 pass · 0 fail · 0 skip	✅ 10 pass · 0 fail · 0 skip
blinker	✅ 25 pass · 0 fail · 0 skip	✅ 25 pass · 0 fail · 0 skip
jinja	✅ 25 pass · 0 fail · 0 skip	✅ 25 pass · 0 fail · 0 skip
installer	✅ 20 pass · 0 fail · 0 skip	✅ 20 pass · 0 fail · 0 skip
tomlkit	✅ 332 pass · 0 fail · 0 skip	✅ 332 pass · 0 fail · 0 skip
outcome	✅ 10 pass · 0 fail · 0 skip	✅ 10 pass · 0 fail · 0 skip
pluggy	✅ 74 pass · 0 fail · 0 skip	✅ 74 pass · 0 fail · 0 skip
werkzeug	✅ 96 pass · 0 fail · 0 skip	✅ 96 pass · 0 fail · 0 skip