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/common/src/last_minute.rs

@@ -13,82 +13,6 @@
 // limitations under the License.
 
 use std::time::{Duration, SystemTime, UNIX_EPOCH};
-
-#[allow(dead_code)]
-#[derive(Debug, Default)]
-struct TimedAction {
-    count: u64,
-    acc_time: u64,
-    min_time: Option<u64>,
-    max_time: Option<u64>,
-    bytes: u64,
-}
-
-#[allow(dead_code)]
-impl TimedAction {
-    // Avg returns the average time spent on the action.
-    pub fn avg(&self) -> Option<Duration> {
-        if self.count == 0 {
-            return None;
-        }
-        Some(Duration::from_nanos(self.acc_time / self.count))
-    }
-
-    // AvgBytes returns the average bytes processed.
-    pub fn avg_bytes(&self) -> u64 {
-        if self.count == 0 {
-            return 0;
-        }
-        self.bytes / self.count
-    }
-
-    // Merge other into t.
-    pub fn merge(&mut self, other: TimedAction) {
-        self.count += other.count;
-        self.acc_time += other.acc_time;
-        self.bytes += other.bytes;
-
-        if self.count == 0 {
-            self.min_time = other.min_time;
-        }
-        if let Some(other_min) = other.min_time {
-            self.min_time = self.min_time.map_or(Some(other_min), |min| Some(min.min(other_min)));
-        }
-
-        self.max_time = self
-            .max_time
-            .map_or(other.max_time, |max| Some(max.max(other.max_time.unwrap_or(0))));
-    }
-}
-
-#[allow(dead_code)]
-#[derive(Debug)]
-enum SizeCategory {
-    SizeLessThan1KiB = 0,
-    SizeLessThan1MiB,
-    SizeLessThan10MiB,
-    SizeLessThan100MiB,
-    SizeLessThan1GiB,
-    SizeGreaterThan1GiB,
-    // Add new entries here
-    SizeLastElemMarker,
-}
-
-impl std::fmt::Display for SizeCategory {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        let s = match *self {
-            SizeCategory::SizeLessThan1KiB => "SizeLessThan1KiB",
-            SizeCategory::SizeLessThan1MiB => "SizeLessThan1MiB",
-            SizeCategory::SizeLessThan10MiB => "SizeLessThan10MiB",
-            SizeCategory::SizeLessThan100MiB => "SizeLessThan100MiB",
-            SizeCategory::SizeLessThan1GiB => "SizeLessThan1GiB",
-            SizeCategory::SizeGreaterThan1GiB => "SizeGreaterThan1GiB",
-            SizeCategory::SizeLastElemMarker => "SizeLastElemMarker",
-        };
-        write!(f, "{s}")
-    }
-}
-
 #[derive(Clone, Debug, Default, Copy)]
 pub struct AccElem {
     pub total: u64,