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/erasure/coding/encode.rs

@@ -841,6 +841,12 @@ mod tests {
     use std::time::Duration;
     use tokio::io::{AsyncWrite, AsyncWriteExt};
 
+    fn erasure_with_zero_block_size() -> Erasure {
+        let mut erasure = Erasure::default();
+        erasure.data_shards = 1;
+        erasure
+    }
+
     #[derive(Clone, Default)]
     struct DeferredCommitWriter {
         buffered: Vec<u8>,
@@ -1500,7 +1506,7 @@ mod tests {
             HashAlgorithm::HighwayHash256S,
         ))];
 
-        let erasure = Arc::new(Erasure::new(1, 0, 0));
+        let erasure = Arc::new(erasure_with_zero_block_size());
         let reader = tokio::io::BufReader::new(Cursor::new(b"payload".to_vec()));
         let err = erasure
             .encode(reader, &mut writers, 1)
@@ -1660,7 +1666,7 @@ mod tests {
         let writer = DeferredCommitWriter::new(committed);
         let mut writers = vec![Some(bitrot_writer(writer, 16))];
 
-        let erasure = Arc::new(Erasure::new(1, 0, 0));
+        let erasure = Arc::new(erasure_with_zero_block_size());
         let reader = tokio::io::BufReader::new(Cursor::new(b"payload".to_vec()));
         let err = erasure
             .encode_batched(reader, &mut writers, 1)