feat: improve object pool

- improve object pool
- add docstring for test coverage

omrf/tests/last_error.rs

@@ -27,6 +27,7 @@ fn parse_cstr(ptr: CStrPtr) -> String {
         .into_owned()
 }
 
+/// Coverage: the cleared/default state -- no outcome recorded, code is `CERROR_OK`, message empty.
 #[test]
 fn fresh_state_is_absolute_success() {
     last_error::clear_last_error();
@@ -35,6 +36,8 @@ fn fresh_state_is_absolute_success() {
     assert_eq!(parse_cstr(last_error::get_error_message()), "");
 }
 
+/// Coverage: `set_last_error` records both the `Into<CError>`-mapped code and the `Display`-derived
+/// message, and `has_last_message` becomes true.
 #[test]
 fn set_records_code_and_message() {
     last_error::clear_last_error();
@@ -44,6 +47,8 @@ fn set_records_code_and_message() {
     assert_eq!(parse_cstr(last_error::get_error_message()), "boom error");
 }
 
+/// Coverage: `clear_last_error` resets a previously-recorded outcome back to the absolute-success
+/// state (no code, empty message).
 #[test]
 fn clear_resets_to_absolute_success() {
     last_error::set_last_error(TestErr::Boom);
@@ -54,6 +59,8 @@ fn clear_resets_to_absolute_success() {
     assert_eq!(parse_cstr(last_error::get_error_message()), "");
 }
 
+/// Coverage: interior NUL bytes in an error's `Display` text are replaced so the message C string
+/// builds without panicking, and the code is still recorded.
 #[test]
 fn interior_nul_in_message_is_sanitized() {
     last_error::clear_last_error();
@@ -65,6 +72,7 @@ fn interior_nul_in_message_is_sanitized() {
     assert!(msg.contains("value"));
 }
 
+/// Coverage: a second `set_last_error` fully overwrites the previously recorded code and message.
 #[test]
 fn overwriting_replaces_previous_state() {
     last_error::clear_last_error();