feat(horses-service): remove integration tests and obsolete configurations

- Deleted outdated integration test classes (`HorseServiceIntegrationTest`, `TransactionalContextTest`, and others) and test resources (`logback-test.xml`).
- Removed obsolete Gradle dependencies related to these tests and revised project module references.
- Simplified `DomPferd` domain model with minor refactorings for serialization and validation.

Signed-off-by: Stefan Mogeritsch <stefan.mo.co@gmail.com>

backend/services/horses/horses-service/build.gradle.kts

@@ -14,10 +14,10 @@ dependencies {
     implementation(projects.platform.platformDependencies)
     implementation(projects.core.coreDomain)
     implementation(projects.core.coreUtils)
-    implementation(projects.horses.horsesDomain)
+  implementation(projects.backend.services.horses.horsesDomain)
   // horses-common: ON HOLD – veraltete API-Referenzen
-  // implementation(projects.horses.horsesCommon)
-    implementation(projects.horses.horsesInfrastructure)
+  // implementation(projects.backend.services.horses.horsesCommon)
+  implementation(projects.backend.services.horses.horsesInfrastructure)
 
     // Spring Boot Starters
     implementation(libs.spring.boot.starter.web)
@@ -28,9 +28,11 @@ dependencies {
     implementation(libs.exposed.core)
     implementation(libs.exposed.dao)
     implementation(libs.exposed.jdbc)
+  implementation(libs.exposed.migration.jdbc)
     implementation(libs.exposed.kotlin.datetime)
     implementation(libs.hikari.cp)
-    runtimeOnly(libs.postgresql.driver)
+  testImplementation(project(":backend:infrastructure:messaging:messaging-client"))
+  runtimeOnly(libs.postgresql.driver)
     testRuntimeOnly(libs.h2.driver)
 
     // Testing