test(ecstore): pin error conversion round-trips for heal-matched variants (#6613)

Backlog#1845 step 1 (pure tests, no behavior change): pin the current behavior of every conversion seam an error crosses before heal, replication, or quorum aggregation classifies it, so the later typed-variant and narrow_to_disk() refactors change these expectations deliberately rather than silently.

Covered seams: DiskError <-> StorageError, DiskError <-> node_service wire Error, DiskError/StorageError <-> io::Error (the by-design identity bridge), and StorageError <-> rustfs_filemeta::Error.

Documented lossy edges pinned as-is: SlowDown collapses to TooManyOpenFiles across the disk boundary (StorageFull to DiskFull likewise), the wire Io catch-all re-wraps the rendered message on every hop and drops the io::ErrorKind, and other(format!) messages with per-disk detail fragment reduce_errs quorum buckets while identical messages still bucket together.

Ref rustfs/backlog#1845

crates/ecstore/src/error/mod.rs

@@ -1097,6 +1097,9 @@ pub struct ErrorResponse {
     pub host_id: String,
 }
 
+#[cfg(test)]
+mod conversion_roundtrip_tests;
+
 #[cfg(test)]
 mod tests {
     use super::*;