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/api/mod.rs

@@ -247,7 +247,8 @@ pub mod config {
             CLASS_RRS, CLASS_STANDARD, Config, DEEP_ARCHIVE, DEFAULT_INLINE_BLOCK, DEFAULT_KVS, DEFAULT_RRS_PARITY,
             EXPRESS_ONEZONE, GLACIER, GLACIER_IR, INLINE_BLOCK, INLINE_BLOCK_ENV, INTELLIGENT_TIERING, MIN_PARITY_DRIVES,
             ONEZONE_IA, OPTIMIZE, OPTIMIZE_ENV, OUTPOSTS, RRS, RRS_ENV, SCHEME_PREFIX, SNOW, STANDARD, STANDARD_ENV, STANDARD_IA,
-            StorageClass, default_parity_count, lookup_config, parse_storage_class, validate_parity, validate_parity_inner,
+            StorageClass, default_parity_count, lookup_config, lookup_config_for_pools, parse_storage_class, validate_parity,
+            validate_parity_inner,
         };
     }
 
@@ -310,8 +311,8 @@ pub mod error {
 
 pub mod erasure {
     pub use crate::erasure::coding::{
-        BitrotReader, BitrotWriter, BitrotWriterWrapper, CustomWriter, Erasure, ReedSolomonEncoder, calc_shard_size,
-        calc_shard_size_legacy,
+        BitrotReader, BitrotWriter, BitrotWriterWrapper, CustomWriter, Erasure, ErasureConstructionError, ReedSolomonEncoder,
+        calc_shard_size, calc_shard_size_legacy,
     };
 }