Add Stryker.NET mutation testing

[PeterKruithof]

Summary

• Extract services, data, entities, and background services into JunoBank.Core class library
• Add Stryker.NET mutation testing to CI pipeline (runs in parallel with build/test)
• Mutation reports uploaded as downloadable artifacts on every PR

Why

Stryker.NET can't compile Blazor projects (Razor source generators don't run during Stryker's Roslyn compilation). Extracting the service layer into a plain class library solves this while also improving architecture — business logic is now cleanly separated from the UI.

What changed

• New project: src/JunoBank.Core/ — all services, entities, DbContext, migrations, utils, background services
• JunoBank.Web now references Core, keeps only: Components, Auth (Blazor-specific), BrowserTimeService, Program.cs
• Tests reference Core instead of Web (all 154 pass)
• Namespaces preserved via <RootNamespace>JunoBank.Web</RootNamespace> — zero code changes needed in existing files
• Dockerfile updated to copy Core project
• Solution updated with Core project

Baseline mutation score

• 28.79% (350 killed / 562 tested / 1394 total mutants)
• 664 mutants have no test coverage at all
• This gives us a concrete metric to improve over time

Test plan

• 154 unit tests pass
• 72/73 E2E tests pass (1 known flaky: request-withdrawal timing)
• Stryker runs successfully and produces HTML report
• Docker build includes Core project
• CI mutation testing job completes on this PR

🤖 Generated with Claude Code

[github-actions]

🧬 Mutation Testing

Metric	Value
Score	47.95% ➡️ (0.00% vs baseline)
Killed	337
Survived	134
No Coverage	234
Timeout	2

What does this mean?

Stryker mutates your code (flips conditions, removes lines, changes values) and runs tests against each mutation. Killed = tests caught it (good). Survived = tests missed it (weak spot). No Coverage = no test touches this code.

Download the full HTML report from the Artifacts section below for per-file details.