chore: adjudicate the last 18 bare dead_code allows in the library crates

Finishes backlog#1823 step 10 outside `rustfs/src` and `protocols`: config, s3select-query, common, madmin, heal, ecstore, signer and notify. Stripped first, then clippy asked which the compiler actually missed — 8 of the 18 were inert.

Seven items are deleted, each checked by grep as well as by clippy:

- `common/last_minute.rs`'s private `TimedAction` (with its impl) and `SizeCategory` (with its `Display` impl). The file's public surface — `AccElem`, `LastMinuteLatency` — stays; ecstore consumes it.
- `s3select-query`'s three `with_*` builders. `DefaultLogicalOptimizer::with_optimizer_rules` looks used, but the call in the same file is `SessionStateBuilder::with_optimizer_rules` from DataFusion; the local methods have no callers.
- `heal/manager.rs`'s `contains_key`. Its six apparent references are all `HashMap::contains_key`.

Three keep their code:

- `heal/storage.rs`'s `Test` variant is constructed by the `#[cfg(test)] test()` helper, which the lib target cannot see, so it takes a reasoned allow.
- `signer`'s `STREAMING_PAYLOAD_HDR` and `try_build_chunk_string_to_sign` gain the `_` prefix instead. That file already marks deliberately-unheld code that way — `_STREAMING_TRAILER_HDR`, `_PAYLOAD_CHUNK_SIZE`, and `_try_build_chunk_signature`, which is the only caller of that function. Following the existing convention removes the allow without an attribute.

`protocols` keeps its four; that crate needs `--features swift,sftp` to compile fully and is verified differently. The four `#![allow(dead_code)]` in `e2e_test` are module-root blankets in test-support files, which belong to steps 1-5 rather than step 10.

Refs backlog#1823

crates/madmin/src/service_commands.rs

@@ -19,7 +19,6 @@ use hyper::Uri;
 use crate::{trace::TraceType, utils::parse_duration};
 
 #[derive(Debug, Default)]
-#[allow(dead_code)]
 pub struct ServiceTraceOpts {
     s3: bool,
     internal: bool,
@@ -41,7 +40,6 @@ pub struct ServiceTraceOpts {
     threshold: Duration,
 }
 
-#[allow(dead_code)]
 impl ServiceTraceOpts {
     pub fn trace_types(&self) -> TraceType {
         let mut tt = TraceType::default();