fix: CachingResolver falls back to stale addresses when refresh fails

The stale-while-revalidate logic in CachingResolver#resolve discarded
previousAddresses when the refresh promise completed with a failure,
propagating UnknownHostException instead of returning the cached
addresses. Use orElseSucceed to fall back to the previous addresses
when the refreshed resolution fails.

Author: guizmaii

zio-http/jvm/src/test/scala/zio/http/DnsResolverSpec.scala

@@ -130,6 +130,38 @@ object DnsResolverSpec extends ZIOHttpSpec {
           implementation = TestResolver(),
         ),
       ),
+      test("resolve returns previous addresses when refresh fails") {
+        for {
+          failing    <- Ref.make(false)
+          failSignal <- Promise.make[Nothing, Unit]
+          result     <- {
+            val impl = new DnsResolver {
+              override def resolve(host: String)(implicit trace: Trace): ZIO[Any, UnknownHostException, Chunk[InetAddress]] =
+                failing.get.flatMap {
+                  case true  => failSignal.succeed(()) *> ZIO.fail(new UnknownHostException(host))
+                  case false => ZIO.succeed(Chunk(InetAddress.getByAddress(Array(127, 0, 0, host.stripPrefix("host").toByte))))
+                }
+            }
+            (for {
+              _      <- DnsResolver.resolve("host1")
+              _      <- failing.set(true)
+              _      <- TestClock.adjust(15.seconds)
+              _      <- failSignal.await
+              result <- DnsResolver.resolve("host1")
+            } yield result).provide(
+              DnsResolver.explicit(
+                ttl = 10.seconds,
+                unknownHostTtl = 10.seconds,
+                refreshRate = 1.second,
+                expireAction = ExpireAction.Refresh,
+                implementation = impl,
+              ),
+            )
+          }
+        } yield assertTrue(
+          result.map(_.toString) == Chunk("/127.0.0.1"),
+        )
+      },
     )
 
   private def stringSnapshot(): ZIO[DnsResolver, Nothing, Set[String]] =

zio-http/shared/src/main/scala/zio/http/DnsResolver.scala

@@ -83,7 +83,7 @@ object DnsResolver {
                 case Some(previous) =>
                   (
                     ZIO.ifZIO(entry.resolvedAddresses.isDone)(
-                      onTrue = entry.resolvedAddresses.await,
+                      onTrue = entry.resolvedAddresses.await.orElseSucceed(previous),
                       onFalse = ZIO.succeed(previous),
                     ),
                     entries,