chore: fix rules check snippet to emit an error code

[ematipico]

Summary

This PR fixes the rules_check script to correctly return an error when a snippet emits multiple diagnostics. I updated the no_irregular_whitespace docs because it was silently failing.

Test Plan

CI should pass

[github-actions]

Parser conformance results on

js/262

Test result	main count	This PR count	Difference
Total	48326	48326	0
Passed	47129	47129	0
Failed	1197	1197	0
Panics	0	0	0
Coverage	97.52%	97.52%	0.00%

jsx/babel

Test result	main count	This PR count	Difference
Total	40	40	0
Passed	37	37	0
Failed	3	3	0
Panics	0	0	0
Coverage	92.50%	92.50%	0.00%

symbols/microsoft

Test result	main count	This PR count	Difference
Total	6549	6549	0
Passed	2201	2201	0
Failed	4348	4348	0
Panics	0	0	0
Coverage	33.61%	33.61%	0.00%

ts/babel

Test result	main count	This PR count	Difference
Total	669	669	0
Passed	597	597	0
Failed	72	72	0
Panics	0	0	0
Coverage	89.24%	89.24%	0.00%

ts/microsoft

Test result	main count	This PR count	Difference
Total	18273	18273	0
Passed	14007	14007	0
Failed	4266	4266	0
Panics	0	0	0
Coverage	76.65%	76.65%	0.00%

[codspeed-hq]

CodSpeed Performance Report

Merging #3458 will improve performances by 6.74%

_{Comparing chore/stronger-rule-check (e60abcc) with chore/stronger-rule-check (cd3ba6a)}

Summary

⚡ 1 improvements
✅ 103 untouched benchmarks

Benchmarks breakdown

	Benchmark	chore/stronger-rule-check	chore/stronger-rule-check	Change
⚡	router_14177007973872119684.ts[uncached]	2.5 ms	2.4 ms	+6.74%