docs: say in CONTRIBUTING.md that the mutation tests are disabled

[vbreuss]

CONTRIBUTING.md still claimed that every push to main runs Stryker.NET, but the mutation tests are switched off by DisableMutationTests until Stryker's runner for the Microsoft.Testing.Platform can capture the coverage of the TUnit tests in time, and before that they ran nightly, not on every push.

Changes

• The mutation test paragraph says that the tests are currently disabled and links the upstream issue stryker-net#3871, like Pipeline/Build.MutationTests.cs and Docs/agents/workflows.md already do.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

Test Results

     92 files       92 suites   2h 18m 45s ⏱️
 38 390 tests  38 389 ✅ 1 💤 0 ❌
223 516 runs  223 515 ✅ 1 💤 0 ❌

Results for commit c625bc8.

[github-actions]

🚀 Benchmark Results

Details

BenchmarkDotNet v0.15.8, Linux Ubuntu 24.04.5 LTS (Noble Numbat)

AMD EPYC 9V74 2.60GHz, 1 CPU, 4 logical and 2 physical cores

.NET SDK 10.0.303

[Host] : .NET 8.0.32 (8.0.32, 8.0.3226.50405), X64 RyuJIT x86-64-v4
Job=InProcess  Toolchain=InProcessEmitToolchain  IterationCount=15

LaunchCount=1  WarmupCount=10

Method	Mean	vs aweXpect	Error	StdDev	Gen0	Gen1	Allocated	vs aweXpect
Contains_aweXpect	278.9 ns		3.22 ns	3.02 ns	0.0443	-	744 B
Contains_FluentAssertions	323.9 ns	+16%	2.74 ns	2.14 ns	0.0958	-	1608 B	+116%
Contains_TUnit	332.9 ns	+19%	5.51 ns	5.16 ns	0.0505	-	848 B	+14%
Equivalency_aweXpect	9,884.3 ns		17.43 ns	16.30 ns	0.2289	-	3880 B
Equivalency_FluentAssertions	257,829.6 ns	+2508%	6,308.22 ns	5,900.71 ns	35.1563	1.9531	590289 B	+15114%
Equivalency_TUnit	14,263.0 ns	+44%	221.03 ns	195.94 ns	0.9308	-	15680 B	+304%
EventTriggered_aweXpect	565.1 ns		9.49 ns	8.41 ns	0.1106	-	1856 B
EventTriggered_FluentAssertions	52,710.7 ns	+9228%	891.17 ns	833.60 ns	0.3052	0.2441	5256 B	+183%
Int_GreaterThan_aweXpect	118.5 ns		0.84 ns	0.79 ns	0.0319	-	536 B
Int_GreaterThan_FluentAssertions	204.5 ns	+73%	2.12 ns	1.77 ns	0.0730	-	1224 B	+128%
Int_GreaterThan_TUnit	199.8 ns	+69%	2.43 ns	2.15 ns	0.0463	-	776 B	+45%
ItemsCount_AtLeast_aweXpect	212.0 ns		3.25 ns	3.04 ns	0.0358	-	600 B
ItemsCount_AtLeast_FluentAssertions	413.2 ns	+95%	6.11 ns	5.42 ns	0.1197	-	2008 B	+235%
ItemsCount_AtLeast_TUnit	302.0 ns	+42%	5.05 ns	4.72 ns	0.0634	-	1064 B	+77%
Multiple_aweXpect	607.6 ns		10.89 ns	10.19 ns	0.1001	-	1688 B
Multiple_FluentAssertions	1,578.7 ns	+160%	37.61 ns	35.18 ns	0.4025	-	6744 B	+300%
Multiple_TUnit	728.4 ns	+20%	9.80 ns	9.17 ns	0.1335	-	2240 B	+33%
Satisfies_aweXpect	140.5 ns		1.47 ns	1.37 ns	0.0329	-	552 B
Satisfies_FluentAssertions	14,909.9 ns	+10514%	267.73 ns	237.33 ns	0.3357	0.2899	5615 B	+917%
Satisfies_TUnit	181.9 ns	+29%	1.33 ns	1.18 ns	0.0424	-	712 B	+29%
String_aweXpect	201.7 ns		1.62 ns	1.44 ns	0.0343	-	576 B
String_FluentAssertions	420.8 ns	+109%	5.67 ns	5.30 ns	0.1373	-	2304 B	+300%
String_TUnit	263.3 ns	+31%	2.71 ns	2.53 ns	0.0544	-	912 B	+58%
StringArray_aweXpect	484.2 ns		5.16 ns	4.83 ns	0.0515	-	872 B
StringArray_FluentAssertions	1,092.7 ns	+126%	12.67 ns	11.23 ns	0.2480	-	4152 B	+376%
StringArray_TUnit	502.5 ns	+4%	5.96 ns	5.57 ns	0.0753	-	1264 B	+45%
StringArrayInAnyOrder_aweXpect	489.9 ns		8.19 ns	7.66 ns	0.0544	-	920 B
StringArrayInAnyOrder_FluentAssertions	15,934.6 ns	+3152%	347.95 ns	325.47 ns	1.9836	0.0610	33471 B	+3538%
StringArrayInAnyOrder_TUnit	585.3 ns	+19%	6.73 ns	6.29 ns	0.0839	-	1408 B	+53%
Throws_aweXpect	8,518.9 ns		29.73 ns	26.35 ns	0.0916	-	1736 B
Throws_FluentAssertions	26,846.7 ns	+215%	165.93 ns	155.21 ns	1.0986	-	18609 B	+972%
Throws_TUnit	8,197.1 ns	-4%	12.55 ns	11.12 ns	0.1068	-	1824 B	+5%
ThrowsAsync_aweXpect	8,082.9 ns		12.80 ns	10.69 ns	0.1221	-	2216 B
ThrowsAsync_FluentAssertions	38,589.4 ns	+377%	150.69 ns	140.96 ns	1.4038	-	23553 B	+963%
ThrowsAsync_TUnit	16,035.1 ns	+98%	49.37 ns	46.18 ns	0.1526	-	2920 B	+32%