ci: close the two missed cargo-mutants in src/builder.rs

Shard 0 of the nightly mutation run reported two MISSED mutants:

  src/builder.rs:178  Builder::failure_retry_period -> Self
  src/builder.rs:210  Builder::dht_backend          -> Self

Both replaced the setter body with `Default::default()`, wiping the
receiver's accumulated state. The existing #[cfg(test)] mod tests has
one of these `<setter>_setter_stores_exact_value` pinning tests for
nearly every other Builder method, but these two were missing:

  - failure_retry_period: no test exercised the setter at all.
  - dht_backend: the method and its callers (build_wait_none_does_not_block
    et al.) are gated `#[cfg(feature = "test-support")]`, but
    cargo-mutants ran with default features only, so neither the method
    nor its tests were ever compiled — the mutation survived trivially.

Fixes:
  - Add `failure_retry_period_setter_stores_exact_value` matching the
    pattern of the other setter tests.
  - Add `dht_backend_setter_stores_value` (gated on `test-support`).
  - Set `additional_cargo_args = ["--features", "test-support"]` in
    .cargo/mutants.toml so cfg-gated setters are part of the mutation
    surface.

Verified locally: `cargo mutants --file src/builder.rs` → 17 mutants,
16 caught, 1 unviable, 0 missed.

.cargo/mutants.toml

@@ -7,6 +7,12 @@ test_tool = "nextest"
 # Async tests + crypto work takes longer under mutation load.
 timeout_multiplier = 3.0
 
+# Enable `test-support` so cfg-gated setters (e.g. Builder::dht_backend) and
+# the integration tests that exercise them are part of the mutation surface.
+# Without this, dht_backend mutations always "survive" trivially because the
+# method isn't compiled in.
+additional_cargo_args = ["--features", "test-support"]
+
 # Skip paths that either can't be meaningfully mutated or are covered by
 # other tools:
 #   - sim_impl.rs: simulator is exercised via the simulator example, not

src/builder.rs

@@ -470,6 +470,28 @@ mod tests {
         assert_eq!(Builder::default().heal_period(d).heal_period, d);
     }
 
+    #[test]
+    fn failure_retry_period_setter_stores_exact_value() {
+        let d = Duration::from_millis(42);
+        assert_eq!(
+            Builder::default().failure_retry_period(d).failure_retry_period,
+            d
+        );
+    }
+
+    #[test]
+    #[cfg(feature = "test-support")]
+    fn dht_backend_setter_stores_value() {
+        use std::sync::Arc;
+
+        use crate::sim::InMemoryDht;
+        let dht = Arc::new(InMemoryDht::new());
+        let before = Builder::default();
+        let after = Builder::default().dht_backend(dht);
+        assert!(before.dht_backend.is_none());
+        assert!(after.dht_backend.is_some());
+    }
+
     #[test]
     fn jitter_setter_stores_exact_value() {
         let b = Builder::default().jitter(0.25);