chore(ci): Bom-Content-Test checks local Maven cache first

[lqiu96]

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 and validate-bom canary project) 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 works with standard GHA runner setups (which use ~/.m2/repository by default, cached by actions/setup-java@v4 with cache: 'maven') and also respects custom local repository locations if maven.repo.local is set. This removes the need for conditional flags in GHA workflows.

[gemini-code-assist]

Code Review

This pull request introduces a local cache check to skip Maven Central reachability verification for artifacts that are already present in the local Maven repository cache, which is particularly useful during release PRs. The review feedback suggests defensively handling cases where the user.home system property is null to prevent constructing an invalid path like "null/.m2/repository".

[gemini-code-assist]

If the user.home system property is not set (which can happen in certain restricted or sandboxed environments), System.getProperty("user.home") will return null. Appending it to a string results in "null/.m2/repository", which will point to an unintended relative path. It is safer to check if userHome is null and handle it defensively.

    String localRepository = System.getProperty("maven.repo.local");
    if (localRepository == null) {
      String userHome = System.getProperty("user.home");
      if (userHome == null) {
        return false;
      }
      // Standard default location for Maven local repository.
      localRepository = userHome + "/.m2/repository";
    }