test(ecstore): assert the error conversions, and stop the census over-reporting

[overtrue]

What

Fourth batch of backlog#1836, taking on the largest remaining cluster: 17 candidates in ecstore. One was a real assertion-less test. The other sixteen were the census mis-reading three legitimate patterns, so this fixes the script rather than the tests — the same call batch two made when its delegation heuristic mis-fired.

The sixteen false positives

#[should_panic(expected = "...")] — 5 tests. should_panic was already listed in VERIFY_SIGNALS, but the check only ran against the function body. The attribute block was collected into attrs and then never used, so the expected panic message — which is the assertion — was invisible to the scan.

A body that is a single call — 9 tests. run(DurabilityMode::Strict).await, aborting_encode_drops_blocked_producer(EncodePipeline::Vec).await. The delegation rule keyed off the callee's name (assert_, verify_, run_, _harness, …), which these do not match. But a body whose entire content is one call delegates by construction, whatever the callee happens to be called — that is now its own rule.

Compile-time contracts — 2 tests. assert_replication_config_ext::<ReplicationConfiguration>() does match the assert_ prefix, but the turbofish sits between the name and the parens and broke \s*\(. And tier.rs's signature guard declares a nested fn and then discards it with let _ = call_legacy_refresh; — the same "the type system is the assertion" shape as the already-recognised fn _name() form.

Tree-wide candidates drop from 53 to 33; ecstore from 17 to 1.

The one that was real

test_error_conversions ran two conversions and threw both results away:

let io_error = std::io::Error::new(std::io::ErrorKind::NotFound, "test");
let _disk_error: DiskError = io_error.into();

It now pins what each conversion owes its caller:

• a plain io::Error becomes DiskError::Io and keeps its kind — it must not be promoted to FileNotFound on the strength of ErrorKind::NotFound, since reduce_errs counts those as different errors during quorum aggregation;
• a typed DiskError boxed through io::Error round-trips back to itself rather than degrading to Io, which is the behaviour the From impl's own comment calls out;
• a serde_json::Error folds into other with its message intact.

Verification

cargo nextest run -p rustfs-ecstore --lib -E 'test(test_error_conversions)' passes, cargo clippy -p rustfs-ecstore --all-targets -- -D warnings is clean, cargo fmt --all applied. The script change was checked against the excluded tests individually — each of the sixteen is confirmed to carry verification the scan could not see.

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