Prefer OpenTelemetryAssertions.assertThat over dual static imports

OpenTelemetryAssertions extends org.assertj.core.api.Assertions, so all
AssertJ assertThat overloads are already in scope via inheritance when
OpenTelemetryAssertions.assertThat is statically imported. Importing both
makes call sites harder to read and can become ambiguous if overloads
overlap.

Add a style-guide note and remove the redundant Assertions.assertThat
static import from the three files currently doing both.

Author: trask

docs/contributing/style-guide.md

@@ -203,6 +203,10 @@ affect end users.
 Prefer AssertJ assertions over JUnit assertions (assertEquals, assertTrue, etc.) for better
 error messages.
 
+In test files that use `io.opentelemetry.sdk.testing.assertj.OpenTelemetryAssertions.assertThat`,
+do not also statically import `org.assertj.core.api.Assertions.assertThat` — `OpenTelemetryAssertions`
+extends `Assertions`, so all AssertJ `assertThat` overloads are already in scope via inheritance.
+
 ### JUnit
 
 Test classes and test methods should generally be package-protected (no explicit visibility

instrumentation/kubernetes-client-7.0/javaagent/src/test/java/io/opentelemetry/javaagent/instrumentation/kubernetesclient/v7_0/KubernetesClientTest.java

@@ -17,7 +17,6 @@
 import static io.opentelemetry.semconv.ServerAttributes.SERVER_PORT;
 import static io.opentelemetry.semconv.UrlAttributes.URL_FULL;
 import static java.util.concurrent.TimeUnit.SECONDS;
-import static org.assertj.core.api.Assertions.assertThat;
 
 import io.kubernetes.client.openapi.ApiCallback;
 import io.kubernetes.client.openapi.ApiClient;

instrumentation/kubernetes-client-7.0/javaagent/src/version20Test/java/io/opentelemetry/javaagent/instrumentation/kubernetesclient/v7_0/KubernetesClientVer20Test.java

@@ -17,7 +17,6 @@
 import static io.opentelemetry.semconv.ServerAttributes.SERVER_PORT;
 import static io.opentelemetry.semconv.UrlAttributes.URL_FULL;
 import static java.util.concurrent.TimeUnit.SECONDS;
-import static org.assertj.core.api.Assertions.assertThat;
 
 import io.kubernetes.client.openapi.ApiCallback;
 import io.kubernetes.client.openapi.ApiClient;

testing-common/src/main/java/io/opentelemetry/instrumentation/testing/junit/http/AbstractHttpServerTest.java

@@ -42,7 +42,6 @@
 import static java.nio.charset.StandardCharsets.US_ASCII;
 import static java.util.Collections.singletonList;
 import static java.util.concurrent.TimeUnit.SECONDS;
-import static org.assertj.core.api.Assertions.assertThat;
 import static org.junit.jupiter.api.Assumptions.assumeFalse;
 import static org.junit.jupiter.api.Assumptions.assumeTrue;