docs: add mutation testing badge

[vbreuss]

This PR adds the Stryker mutation testing badge to the README, that was removed in #297, enhancing the project's quality metrics visibility alongside existing build, quality gate, and coverage badges.

[Copilot]

Pull request overview

This PR adds a Stryker mutation testing badge to the README, enhancing the project's quality metrics visibility alongside existing build, quality gate, and coverage badges.

Key changes:

• Added mutation testing badge from Stryker Mutator dashboard

[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

    14 files  ±0      14 suites  ±0   5m 29s ⏱️ -22s
 1 802 tests ±0   1 801 ✅ ±0  1 💤 ±0  0 ❌ ±0 
12 031 runs  ±0  12 030 ✅ ±0  1 💤 ±0  0 ❌ ±0 

Results for commit 1558149. ± Comparison against base commit 15c4614.

[github-actions]

🚀 Benchmark Results

Details

BenchmarkDotNet v0.15.8, Linux Ubuntu 24.04.3 LTS (Noble Numbat)

AMD EPYC 7763 2.45GHz, 1 CPU, 4 logical and 2 physical cores

.NET SDK 10.0.101

[Host] : .NET 10.0.1 (10.0.1, 10.0.125.57005), X64 RyuJIT x86-64-v3
Job=InProcess  Toolchain=InProcessEmitToolchain  IterationCount=15

LaunchCount=1  WarmupCount=10

Method	Mean	Error	StdDev	Gen0	Gen1	Allocated
Simple_Mockolate	1.585 μs	0.0051 μs	0.0046 μs	0.2270	-	3.72 KB
Simple_Moq	177.446 μs	0.3800 μs	0.3173 μs	0.4883	-	14.77 KB
Simple_NSubstitute	5.598 μs	0.0116 μs	0.0103 μs	0.5569	0.0076	9.14 KB
Simple_FakeItEasy	6.301 μs	0.0276 μs	0.0258 μs	0.4959	-	8.11 KB

[github-actions]

This is addressed in release v0.49.0.