chore(tests+config): enhance EntriesIsolationIntegrationTest and add missing Spring metadata

- Improved schema isolation logic with constants for tenant schemas and search path management in PostgreSQL.
- Added `withTenant` utility in `TenantContextHolder` to simplify tenant context usage.
- Removed unused imports, variables, and helper functions (`random()` and redundant `NennungRepository` references).
- Included missing `multitenancy.*` configuration keys in `additional-spring-configuration-metadata.json` to address IDE warnings.

backend/services/entries/entries-service/src/main/resources/META-INF/additional-spring-configuration-metadata.json

@@ -0,0 +1,16 @@
+{
+  "properties": [
+    {
+      "name": "multitenancy.registry.type",
+      "type": "java.lang.String",
+      "description": "Type of tenant registry (jdbc or inmem).",
+      "defaultValue": "jdbc"
+    },
+    {
+      "name": "multitenancy.defaultSchemas",
+      "type": "java.lang.String",
+      "description": "Comma-separated list of default schemas for inmem registry.",
+      "defaultValue": "public"
+    }
+  ]
+}