feat(validation): integrate ÖTO/FEI rule validations and centralized validators

- Added `OetoValidators` with live-validation for OEPS numbers, FEI-IDs, license classes, and horse data to align with ÖTO/FEI 2026 standards.
- Expanded `ReiterProfilViewModel` and `PferdProfilViewModel` to include validation states (`ValidationResult`) for enhanced form feedback and dirty state tracking.
- Standardized mock data in `Stores.kt` and `NennungModels.kt` to comply with updated validation rules.
- Created `OetoValidatorsTest` to ensure validation logic accuracy (30 unit tests, all green).
- Updated `build.gradle.kts` to include `kotlin.test` dependency for JVM testing.

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

frontend/core/domain/build.gradle.kts

@@ -19,6 +19,9 @@ kotlin {
     commonMain.dependencies {
       implementation(libs.kotlinx.serialization.json)
     }
+    jvmTest.dependencies {
+      implementation(libs.kotlin.test)
+    }
   }
 }