Implement Stryker mutation‑testing quality gate

[AugistineCreates]

This pr closes #914

Implement Stryker mutation‑testing quality gate

• Add Stryker configuration to run mutation testing on src/services/retry.ts and src/services/fraud.ts.
• Include scripts test:mutation (full run) and test:mutation:dry (dry‑run) in package.json.
• Set mutation thresholds (high ≥ 80%, low ≥ 70%, break ≥ 80%).
• Configure HTML reporter to output results to reports/mutation/html/index.html.
• Ensure type checks are disabled for faster runs.

This change adds a quality‑gate that will alert the team when mutation scores drop below the defined thresholds, helping catch weak test assertions early.

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