fix(ci): check local cache before Maven Central in reachability test

Modify BomContentTest to check if an artifact exists in the local Maven
repository (.m2) before checking its reachability on Maven Central.

During release PRs, new versions of libraries are built and installed locally
(via pre-install.sh) but are not yet published to Maven Central. This
change allows the reachability check to pass for these new versions if they
are found in the local cache, while still verifying that other dependencies
(which should be on Central) are reachable.

This removes the need for conditional flags in GHA workflows.

Author: lqiu96

java-cloud-bom/tests/src/test/java/com/google/cloud/BomContentTest.java

@@ -30,6 +30,7 @@
 import java.io.IOException;
 import java.net.HttpURLConnection;
 import java.net.URL;
+import java.nio.file.Files;
 import java.nio.file.Path;
 import java.nio.file.Paths;
 import java.util.ArrayList;
@@ -76,6 +77,9 @@ private void checkBom(Path bomPath) throws Exception {
       if (artifact.getVersion().contains("SNAPSHOT")) {
         continue;
       }
+      if (existsLocally(artifact)) {
+        continue;
+      }
       assertReachable(buildMavenCentralUrl(artifact));
     }
 
@@ -104,6 +108,9 @@ static void checkBomReachable(Path bomPath) throws Exception {
       if (artifact.getVersion().contains("SNAPSHOT")) {
         continue;
       }
+      if (existsLocally(artifact)) {
+        continue;
+      }
       try {
         assertReachable(buildMavenCentralUrl(artifact));
       } catch (IOException ex) {
@@ -134,6 +141,19 @@ private static String buildMavenCentralUrl(Artifact artifact) {
         + ".pom";
   }
 
+  private static boolean existsLocally(Artifact artifact) {
+    String localRepository = System.getProperty("maven.repo.local");
+    if (localRepository == null) {
+      localRepository = System.getProperty("user.home") + "/.m2/repository";
+    }
+    Path localPath = Paths.get(localRepository,
+        artifact.getGroupId().replace('.', '/'),
+        artifact.getArtifactId(),
+        artifact.getVersion(),
+        artifact.getArtifactId() + "-" + artifact.getVersion() + ".pom");
+    return Files.exists(localPath);
+  }
+
   /** Asserts that the BOM only provides JARs which contains unique class names to the classpath. */
   private static void assertUniqueClasses(List<Artifact> allArtifacts)
       throws InvalidVersionSpecificationException, IOException {