ci: serialize Memcached and Kafka container tests

Same resource contention fix as AWS SDK tests — add [Collection] to
Memcached and Kafka test classes so each pair of DotNet8/DotNet10 tests
runs sequentially. Prevents two instances of the same dependency service
(memcached-server, kafka-broker) from competing for resources on CI.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: tippmar-nr

tests/Agent/IntegrationTests/ContainerIntegrationTests/Tests/KafkaTests.cs

@@ -134,6 +134,7 @@ internal static string GenerateTopic()
     }
 }
 
+[Collection("KafkaTests")]
 [Trait("Architecture", "amd64")]
 [Trait("Distro", "Ubuntu")]
 public class KafkaDotNet8Test : LinuxKafkaTest<KafkaDotNet8TestFixture>
@@ -143,6 +144,7 @@ public KafkaDotNet8Test(KafkaDotNet8TestFixture fixture, ITestOutputHelper outpu
     }
 }
 
+[Collection("KafkaTests")]
 [Trait("Architecture", "amd64")]
 [Trait("Distro", "Ubuntu")]
 public class KafkaDotNet10Test : LinuxKafkaTest<KafkaDotNet10TestFixture>

tests/Agent/IntegrationTests/ContainerIntegrationTests/Tests/MemcachedTests.cs

@@ -127,6 +127,7 @@ public void Test()
     }
 }
 
+[Collection("MemcachedTests")]
 [Trait("Architecture", "amd64")]
 [Trait("Distro", "Ubuntu")]
 public class MemcachedDotNet8Test : LinuxMemcachedTest<MemcachedDotNet8TestFixture>
@@ -136,6 +137,7 @@ public MemcachedDotNet8Test(MemcachedDotNet8TestFixture fixture, ITestOutputHelp
     }
 }
 
+[Collection("MemcachedTests")]
 [Trait("Architecture", "amd64")]
 [Trait("Distro", "Ubuntu")]
 public class MemcachedDotNet10Test : LinuxMemcachedTest<MemcachedDotNet10TestFixture>