fix: prevent session pool deadlock on acquire cancellation

Replace is_creating mutex with atomic flag and RAII guard to ensure
proper cleanup when acquire() futures are cancelled mid-creation,
preventing permanent deadlock for subsequent callers.

tests/db_batch_upsert.rs

@@ -1,3 +1,4 @@
+#[allow(dead_code)]
 mod helpers;
 
 use banner::data::batch::batch_upsert_courses;