fix(ml_training): mark DB-dependent test as #[ignore]

test_semantic_version_validation requires DATABASE_URL env var pointing
to a live PostgreSQL instance. Mark as ignored so CI doesn't fail.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

services/ml_training_service/src/checkpoint_manager.rs

@@ -494,6 +494,7 @@ mod tests {
     use super::*;
 
     #[tokio::test]
+    #[ignore = "requires PostgreSQL (DATABASE_URL)"]
     async fn test_semantic_version_validation() {
         let pool = PgPool::connect(&std::env::var("DATABASE_URL").unwrap())
             .await