fix: resolve all clippy warnings for CI (async_fn_in_trait, collapsible_if, type_complexity, dead_code)

- Allow async_fn_in_trait lint crate-wide (internal project, 413 warnings)
- Add integration_tests feature to Cargo.toml to fix unexpected cfg warnings
- Collapse nested if statements into single conditions (13 locations)
- Add type_complexity allows on pg repository functions (12 locations)
- Fix dead code warnings in test modules with allow attributes
- Fix E0599 by gating new_stub() for integration_tests feature
- Add result_unit_err and result_large_err allows where appropriate
- Apply rustfmt formatting

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

src/application/services/batch_operations.rs

@@ -1041,10 +1041,15 @@ impl BatchOperationService {
 
 #[cfg(integration_tests)]
 mod tests {
+    #[allow(unused_imports)]
     use super::*;
+    #[allow(unused_imports)]
     use crate::infrastructure::repositories::pg::file_blob_read_repository::FileBlobReadRepository;
+    #[allow(unused_imports)]
     use crate::infrastructure::repositories::pg::file_blob_write_repository::FileBlobWriteRepository;
+    #[allow(unused_imports)]
     use crate::infrastructure::repositories::pg::folder_db_repository::FolderDbRepository;
+    #[allow(unused_imports)]
     use std::sync::Arc;
 
     #[tokio::test]