fix(ecstore): resolve erasure parity per pool (#4801) (#5015)

* fix(ecstore): add fallible erasure construction

(cherry picked from commit bd148b20f7)

* fix(ecstore): resolve storage parity per pool

(cherry picked from commit c05c2cb24b)

* fix(ecstore): keep carved per-pool parity core self-contained on main

Fixups so the cherry-picked fallible-erasure + per-pool-parity core builds standalone on current main without the excluded scope-creep commits:
- runtime/sources.rs: re-add backend_storage_class_parities (removed by the per-pool commit; its rebalance caller was updated in an unrelated reporting commit that was left out). Reimplemented over the snapshot API, behavior-identical.
- config/mod.rs: rename the storage-class publish test module (main independently added a mod tests, so the cherry-pick collided).
- rustfs storage_api.rs + startup_storage.rs: route the storage-class ENV consts through the startup storage facade and use a local const for the erasure-set-drive-count env name, satisfying the layer/facade guardrail (main's guardrail is stricter than when the core was authored).

* fix(ecstore): use struct-init in erasure test helper to satisfy clippy field_reassign_with_default

The cherry-picked fallible-erasure commit's `erasure_with_invalid_dimensions` test helper built `Erasure` via `default()` then reassigned fields, which trips `clippy::field_reassign_with_default` under `-D warnings` (only surfaced by `--all-targets`, which lints test code). #4977 fixed this in a later commit that was not part of the carved core. Use struct-init with `..Default::default()`, matching #4977's final form.

* fix(rustfs): gate the test-only storage-class ENV facade re-export behind cfg(test)

The ENV constants (INLINE_BLOCK_ENV/OPTIMIZE_ENV/RRS_ENV/STANDARD_ENV) re-exported through the startup storage facade are only consumed by a #[cfg(test)] test in startup_storage.rs, so in a non-test lib build the re-export is unused and trips -D unused-imports under clippy --all-targets. Gate it with #[cfg(test)], matching #4977's final form.

---------

Co-authored-by: cxymds <cxymds@gmail.com>

crates/ecstore/src/disk/error.rs

@@ -141,7 +141,7 @@ pub enum DiskError {
     ErasureReadQuorum,
 
     #[error("io error {0}")]
-    Io(io::Error),
+    Io(#[source] io::Error),
 
     #[error("source stalled")]
     SourceStalled,
@@ -153,6 +153,12 @@ pub enum DiskError {
     InvalidPath,
 }
 
+impl From<crate::erasure::coding::ErasureConstructionError> for DiskError {
+    fn from(error: crate::erasure::coding::ErasureConstructionError) -> Self {
+        Self::Io(error.into_io_error())
+    }
+}
+
 impl DiskError {
     pub fn other<E>(error: E) -> Self
     where
@@ -601,6 +607,26 @@ mod tests {
     use super::*;
     use std::collections::HashMap;
 
+    #[test]
+    fn other_preserves_erasure_construction_source_chain() {
+        use crate::erasure::coding::ErasureConstructionError;
+        use std::error::Error as _;
+
+        let error = DiskError::from(ErasureConstructionError::ModernEncoder {
+            source: reed_solomon_erasure::Error::TooManyShards,
+        });
+        let io_source = error.source().expect("DiskError::Io must expose its io::Error source");
+        assert!(io_source.is::<io::Error>());
+        let construction_source = io_source
+            .source()
+            .expect("io::Error must expose the erasure construction error");
+        assert!(construction_source.is::<ErasureConstructionError>());
+        let encoder_source = construction_source
+            .source()
+            .expect("construction error must expose the encoder error");
+        assert!(encoder_source.is::<reed_solomon_erasure::Error>());
+    }
+
     #[test]
     fn test_disk_error_variants() {
         let errors = vec![