chore(test): exclude BeoordelingEscalatiePump from mutation (S-14, refs #15)

The hosted escalation pump is a thin shell with logic in the unit-tested
processor, mirroring the existing OpenZaakJobPump exclusion (§5).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

services/domain/stryker-config.json

@@ -4,7 +4,8 @@
     "test-projects": ["Big.Tests/Big.Tests.csproj"],
     "reporters": ["progress", "html"],
     "mutate": [
-      "!**/OpenZaakJobPump.cs"
+      "!**/OpenZaakJobPump.cs",
+      "!**/BeoordelingEscalatiePump.cs"
     ],
     "thresholds": {
       "high": 95,