Add Stryker.NET mutation testing configuration and CI workflow

Agent-Logs-Url: https://github.com/Kros-sk/TeaPie/sessions/06690327-20a3-4c5b-a653-d6c40201601d

Co-authored-by: Burgyn <5930822+Burgyn@users.noreply.github.com>

Author: Copilot

.config/dotnet-tools.json

@@ -0,0 +1,13 @@
+{
+  "version": 1,
+  "isRoot": true,
+  "tools": {
+    "dotnet-stryker": {
+      "version": "4.14.0",
+      "commands": [
+        "dotnet-stryker"
+      ],
+      "rollForward": false
+    }
+  }
+}

.github/workflows/mutation-testing.yml

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+name: Mutation Testing
+
+on:
+  schedule:
+    - cron: '0 3 * * 1'  # Every Monday at 03:00 UTC
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  mutation-testing:
+    runs-on: ubuntu-latest
+    timeout-minutes: 60
+
+    steps:
+    - name: Checkout code
+      uses: actions/checkout@v4
+
+    - name: Setup .NET
+      uses: actions/setup-dotnet@v3
+      with:
+        dotnet-version: '8.x'
+
+    - name: Restore tools
+      run: dotnet tool restore
+
+    - name: Install dependencies
+      run: dotnet restore
+
+    - name: Run Stryker mutation testing
+      run: dotnet stryker
+
+    - name: Upload mutation report
+      if: always()
+      uses: actions/upload-artifact@v4
+      with:
+        name: mutation-report
+        path: '**/StrykerOutput/**/reports/**'
+        retention-days: 14

.gitignore

@@ -401,6 +401,9 @@ FodyWeavers.xsd
 **/.teapie/cache/
 **/.teapie/reports/
 
+# Stryker mutation testing
+**/StrykerOutput/
+
 # User-defined
 *launchSettings*.json
 docs/_site

docs/mutation-testing.md

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+# Mutation Testing
+
+TeaPie uses [Stryker.NET](https://stryker-mutator.io/docs/stryker-net/introduction/) for mutation testing to verify the effectiveness of the test suite.
+
+## Overview
+
+Mutation testing systematically introduces small changes (*mutants*) into the source code and checks whether the existing tests detect them. A killed mutant means the tests caught the change; a surviving mutant may indicate a gap in test assertions.
+
+## Quick Start
+
+```bash
+# Restore the local tool
+dotnet tool restore
+
+# Run mutation testing
+dotnet stryker
+```
+
+The HTML report is generated in the `StrykerOutput/` directory.
+
+## Configuration
+
+Stryker is configured via [`stryker-config.json`](../stryker-config.json) in the repository root. Key settings:
+
+| Setting | Value | Description |
+|---------|-------|-------------|
+| `project` | `TeaPie.csproj` | Source project under test |
+| `test-projects` | `TeaPie.Tests.csproj` | Test project |
+| `reporters` | html, progress, cleartext | Output formats |
+| `thresholds.high` | 80 | Score above this is green |
+| `thresholds.low` | 60 | Score below this is red |
+
+## CI Integration
+
+Mutation testing runs as a **scheduled weekly workflow** (`.github/workflows/mutation-testing.yml`) every Monday at 03:00 UTC. It can also be triggered manually via `workflow_dispatch`. The HTML report is uploaded as a build artifact.
+
+## Baseline Score
+
+The initial baseline mutation score is **37.83%**, established with Stryker.NET 4.14.0.
+
+| Metric | Count |
+|--------|-------|
+| Killed | 787 |
+| Survived | 707 |
+| Timeout | 6 |
+| No Coverage | 596 |
+| Compile Error | 145 |
+| Ignored | 677 |
+
+The relatively low score is expected for an initial baseline. Many surviving mutants are in code paths not yet covered by tests (No Coverage) or in areas like console rendering and pipeline orchestration that are harder to unit test. The score will improve as tests are strengthened over time.

stryker-config.json

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+{
+  "stryker-config": {
+    "project": "TeaPie.csproj",
+    "test-projects": ["./tests/TeaPie.Tests/TeaPie.Tests.csproj"],
+    "solution": "./TeaPie.sln",
+    "reporters": ["html", "progress", "cleartext"],
+    "thresholds": {
+      "high": 80,
+      "low": 60,
+      "break": 0
+    }
+  }
+}