test(ecstore): assert the error conversions, and stop the census over-reporting

The census listed 17 candidates in ecstore. Sixteen were false positives of three shapes, and reading them showed the heuristics rather than the tests were wrong:

- `#[should_panic(expected = "...")]` (5). `should_panic` was already in the verification signals, but the check only ever ran against the function body — the attribute block was collected and then ignored, so the expected panic message, which *is* the assertion, was invisible.
- Bodies that are a single call into a shared harness (9), like `run(DurabilityMode::Strict).await` and `aborting_encode_drops_blocked_producer(EncodePipeline::Vec).await`. The delegation rule keyed off callee names (`assert_`/`verify_`/`run_`/`_harness`), which these do not match, though a body that is nothing but one call delegates by construction whatever the callee is called.
- Compile-time contracts (2): a turbofish between the callee and its parens (`assert_replication_config_ext::<T>()`) broke the delegation regex, and a nested `fn` that is only bound and discarded is the same signature guard as the already-recognised `fn _name()` form.

The script now folds the attribute block into the verification text, allows a turbofish in the delegation patterns, and recognises both a single-call body and a discarded nested-fn binding. Tree-wide candidates drop from 53 to 33, ecstore from 17 to 1.

The one that survives was real: `test_error_conversions` performed two conversions and discarded both results. It now pins what each conversion must produce — a plain `io::Error` stays `DiskError::Io` rather than being guessed at from its `NotFound` kind, a typed error boxed through `io::Error` round-trips back to itself instead of degrading to `Io`, and a serde_json error folds into `other` with its message intact.

Refs backlog#1836

crates/ecstore/src/disk/error.rs

@@ -853,13 +853,32 @@ mod tests {
 
     #[test]
     fn test_error_conversions() {
-        // Test From implementations
+        // A plain io::Error carries no typed payload to recover, so it lands in
+        // `Io` rather than being guessed at from its kind — `NotFound` here must
+        // not silently become `FileNotFound`, which quorum aggregation counts as
+        // a different error (rustfs/backlog#1836).
         let io_error = std::io::Error::new(std::io::ErrorKind::NotFound, "test");
-        let _disk_error: DiskError = io_error.into();
+        let disk_error: DiskError = io_error.into();
+        match &disk_error {
+            DiskError::Io(inner) => assert_eq!(inner.kind(), std::io::ErrorKind::NotFound),
+            other => panic!("a plain io::Error must stay typed as Io, got {other:?}"),
+        }
 
-        let json_str = r#"{"invalid": json}"#; // Invalid JSON
+        // A typed DiskError boxed through io::Error round-trips back to itself
+        // instead of degrading to `Io`.
+        let boxed: std::io::Error = std::io::Error::other(DiskError::VolumeNotFound);
+        assert_eq!(DiskError::from(boxed), DiskError::VolumeNotFound);
+
+        // serde_json errors have no dedicated variant and fold into `other`,
+        // keeping the original message.
+        let json_str = r#"{"invalid": json}"#;
         let json_error = serde_json::from_str::<serde_json::Value>(json_str).unwrap_err();
-        let _disk_error: DiskError = json_error.into();
+        let json_message = json_error.to_string();
+        let disk_error: DiskError = json_error.into();
+        assert!(
+            disk_error.to_string().contains(&json_message),
+            "the json error message must survive the conversion: {disk_error}"
+        );
     }
 
     #[test]