refactor(ecstore): make store-to-disk error narrowing named and fallible (#6626)

refactor(ecstore): make store-to-disk error narrowing a named fallible operation

Backlog#1845 step 4. The blanket impl From<StorageError> for DiskError let ? silently push store-only errors (locks, buckets, quotas) across the disk boundary into DiskError::other, where the rendered message fragments reduce_errs quorum buckets. Same story for the blanket From<StorageError> for rustfs_filemeta::Error and its other() catch-all.

Both impls are replaced by named, fallible methods: StorageError::narrow_to_disk() and StorageError::narrow_to_filemeta(). Variants with an identity on the far side map across unchanged - including the two documented lossy collapses (SlowDown -> TooManyOpenFiles, StorageFull -> DiskFull) that the round-trip tests pin - and everything else returns Err(self) so the call site decides what crossing the boundary means. Removing the impls let the compiler enumerate every conversion site; the census that scoped this issue had found 5, the compiler found 33.

Call sites keep their existing behavior: the io identity bridge and the generic sites fold Err into the io-backed other() exactly as the old catch-all did (identity still recoverable by downcast), listing paths use one shared to_filemeta_err helper, and the two sites that relied on the SlowDown collapse now construct DiskError::TooManyOpenFiles directly so the loss is visible where it happens. No behavior change intended anywhere; the io::Error bridge itself is untouched by design.

Ref rustfs/backlog#1845

crates/ecstore/src/set_disk/ops/object.rs

@@ -2205,7 +2205,7 @@ impl SetDisks {
         }
         let current = read_object_transaction_epoch_fence(self, bucket, object)
             .await
-            .map_err(DiskError::from)?;
+            .map_err(|e| e.narrow_to_disk().unwrap_or_else(DiskError::other))?;
         let disks = self.get_disks_internal().await;
         let mut removed = 0usize;