[4/n] [reconfigurator] more control over ExampleSystem zone allocation (

#6785)

`ExampleSystem` instances now allow for controlling the number of Nexus
and DNS zones.

There was one test that manually poked at the `ExampleSystem` earlier -- that
test can now switch to using this scheme. (I didn't realize it at the time but
we were allocating just one Nexus zone per sled -- I thought we were doing 3
total. Well, now it's clear how many zones are being allocated.)

Also add tests to ensure zone allocation works properly.

nexus/reconfigurator/planning/src/blueprint_builder/builder.rs

@@ -1646,12 +1646,11 @@ impl<'a> BlueprintBuilder<'a> {
     /// ordinarily only come from RSS.
     ///
     /// TODO-cleanup: Remove when external DNS addresses are in the policy.
-    #[cfg(test)]
-    #[track_caller]
-    pub fn add_external_dns_ip(&mut self, addr: IpAddr) {
-        self.external_networking()
-            .expect("failed to initialize external networking allocator")
-            .add_external_dns_ip(addr);
+    pub(crate) fn add_external_dns_ip(
+        &mut self,
+        addr: IpAddr,
+    ) -> Result<(), Error> {
+        self.external_networking()?.add_external_dns_ip(addr)
     }
 }
 

nexus/reconfigurator/planning/src/blueprint_builder/external_networking.rs

@@ -3,6 +3,7 @@
 // file, You can obtain one at https://mozilla.org/MPL/2.0/.
 
 use super::Error;
+use anyhow::anyhow;
 use anyhow::bail;
 use debug_ignore::DebugIgnore;
 use nexus_config::NUM_INITIAL_RESERVED_IP_ADDRESSES;
@@ -371,12 +372,18 @@ impl<'a> BuilderExternalNetworking<'a> {
     /// which could otherwise only be added via RSS.
     ///
     /// TODO-cleanup: Remove when external DNS addresses are in the policy.
-    #[cfg(test)]
-    pub fn add_external_dns_ip(&mut self, addr: IpAddr) {
-        assert!(
-            self.available_external_dns_ips.insert(addr),
-            "duplicate external DNS IP address"
-        );
+    pub(crate) fn add_external_dns_ip(
+        &mut self,
+        addr: IpAddr,
+    ) -> Result<(), Error> {
+        if self.available_external_dns_ips.contains(&addr) {
+            return Err(Error::Planner(anyhow!(
+                "external DNS IP address already in use: {addr}"
+            )));
+        }
+
+        self.available_external_dns_ips.insert(addr);
+        Ok(())
     }
 }