Add warning assertion helpers

[MatthewMckee4]

Summary

Add typed karva.warns and karva.deprecated_call context managers with regex matching, nested warning handling, filter restoration, and recorded warning access. Document their pytest-compatible use. Closes #955.

Test Plan

Ran just test -p karva fixture_support, built the docs, and ran uvx prek run -a.

[github-actions]

❌ Merging this PR may alter performance

Baseline: main (ce8ff7d). Candidate: PR (ecccfd9). 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.

⚡ 1 improved benchmark
❌ 2 regressed benchmarks
✅ 11 unchanged benchmarks

Warning

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

Performance Changes

	Mode	Benchmark	Base	Head	Change	Runs
❌	WallTime	h11	398.8 ms	408.5 ms	+2.4%	21
❌	WallTime	installer	286.2 ms	295.3 ms	+3.2%	21
⚡	WallTime	tomlkit	541.1 ms	523.7 ms	-3.2%	15

All benchmark scores

	Mode	Benchmark	Base	Head	Change	Runs
✅	WallTime	requests	16.051 s	16.061 s	+0.1%	4
✅	WallTime	fastapi	6.187 s	6.196 s	+0.1%	7
✅	WallTime	httpx	7.540 s	7.542 s	+0.0%	7
❌	WallTime	h11	398.8 ms	408.5 ms	+2.4%	21
✅	WallTime	markupsafe	253.4 ms	253.4 ms	+0.0%	21
✅	WallTime	sniffio	268.3 ms	269.1 ms	+0.3%	21
✅	WallTime	itsdangerous	255.7 ms	256.0 ms	+0.1%	21
✅	WallTime	blinker	284.9 ms	285.5 ms	+0.2%	21
✅	WallTime	jinja	574.5 ms	574.3 ms	-0.0%	21
❌	WallTime	installer	286.2 ms	295.3 ms	+3.2%	21
⚡	WallTime	tomlkit	541.1 ms	523.7 ms	-3.2%	15
✅	WallTime	outcome	269.8 ms	270.3 ms	+0.2%	21
✅	WallTime	pluggy	348.1 ms	348.2 ms	+0.0%	21
✅	WallTime	werkzeug	8.677 s	8.640 s	-0.4%	7

[github-actions]

✅ Merging this PR will not alter memory usage

All benchmark scores

	Mode	Benchmark	Base	Head	Change	Runs
✅	Memory	requests	60.2 MiB	60.1 MiB	-0.0%	4
✅	Memory	fastapi	146.0 MiB	146.5 MiB	+0.3%	7
✅	Memory	httpx	81.0 MiB	81.0 MiB	+0.0%	7
✅	Memory	h11	53.1 MiB	53.1 MiB	+0.1%	21
✅	Memory	markupsafe	41.9 MiB	41.9 MiB	-0.1%	21
✅	Memory	sniffio	47.5 MiB	47.6 MiB	+0.2%	21
✅	Memory	itsdangerous	44.1 MiB	44.2 MiB	+0.1%	21
✅	Memory	blinker	46.9 MiB	46.9 MiB	+0.0%	21
✅	Memory	jinja	56.9 MiB	56.9 MiB	-0.1%	21
✅	Memory	installer	48.7 MiB	48.8 MiB	+0.1%	21
✅	Memory	tomlkit	49.0 MiB	49.1 MiB	+0.1%	15
✅	Memory	outcome	48.5 MiB	48.6 MiB	+0.1%	21
✅	Memory	pluggy	45.9 MiB	45.9 MiB	+0.1%	21
✅	Memory	werkzeug	136.3 MiB	135.8 MiB	-0.4%	7

[github-actions]

✅ No diagnostic changes

Test result comparison

Project	Previous	New
requests	❌ 149 pass · 0 fail · 6 error · 12 skip	❌ 149 pass · 0 fail · 6 error · 12 skip
fastapi	❌ 1596 pass · 7 fail · 434 error · 1 skip	❌ 1596 pass · 7 fail · 434 error · 1 skip
httpx	❌ 1022 pass · 50 fail · 54 error · 1 skip	❌ 1022 pass · 50 fail · 54 error · 1 skip
h11	✅ 78 pass · 0 fail · 0 error · 0 skip	✅ 78 pass · 0 fail · 0 error · 0 skip
markupsafe	❌ 0 pass · 0 fail · 26 error · 0 skip	❌ 0 pass · 0 fail · 26 error · 0 skip
sniffio	✅ 3 pass · 0 fail · 0 error · 1 skip	✅ 3 pass · 0 fail · 0 error · 1 skip
itsdangerous	✅ 10 pass · 0 fail · 0 error · 0 skip	✅ 10 pass · 0 fail · 0 error · 0 skip
blinker	✅ 25 pass · 0 fail · 0 error · 0 skip	✅ 25 pass · 0 fail · 0 error · 0 skip
jinja	❌ 135 pass · 0 fail · 16 error · 0 skip	❌ 135 pass · 0 fail · 16 error · 0 skip
installer	✅ 20 pass · 0 fail · 0 error · 0 skip	✅ 20 pass · 0 fail · 0 error · 0 skip
tomlkit	✅ 332 pass · 0 fail · 0 error · 0 skip	✅ 332 pass · 0 fail · 0 error · 0 skip
outcome	✅ 10 pass · 0 fail · 0 error · 0 skip	✅ 10 pass · 0 fail · 0 error · 0 skip
pluggy	❌ 124 pass · 1 fail · 27 error · 0 skip	❌ 124 pass · 1 fail · 27 error · 0 skip
werkzeug	❌ 728 pass · 3 fail · 0 error · 0 skip	❌ 728 pass · 3 fail · 0 error · 0 skip