Add external cluster support to skip ephemeral startup

[Mpdreamz]

Adds the ability to skip starting a local ephemeral Elasticsearch cluster
when a remote cluster is already available. This dramatically reduces
integration test iteration time during development (no ~60s bootstrap wait).

How it works

ElasticsearchCluster<TConfiguration>.InitializeAsync() now checks for an
external cluster before launching an ephemeral one. Resolution order:

1. TryUseExternalCluster() virtual method (programmatic hook)
2. TEST_ELASTICSEARCH_URL environment variable
3. Fall through to ephemeral startup (existing behavior)

Environment variable usage (zero code changes)

# Point at a running cluster — no code changes needed
TEST_ELASTICSEARCH_URL=https://localhost:9200 dotnet run --project MyTests/

# With API key authentication
TEST_ELASTICSEARCH_URL=https://localhost:9200 \
TEST_ELASTICSEARCH_API_KEY=your-api-key-here \
dotnet run --project MyTests/

Programmatic hook

Override TryUseExternalCluster() for custom logic (service discovery,
config files, per-developer overrides, etc.):

public class MyTestCluster : ElasticsearchCluster
{
    public MyTestCluster() : base(new ElasticsearchConfiguration("latest-9")) { }

    protected override ExternalClusterConfiguration TryUseExternalCluster()
    {
        var url = Environment.GetEnvironmentVariable("MY_DEV_CLUSTER_URL");
        if (string.IsNullOrEmpty(url))
            return null; // fall through to ephemeral startup

        return new ExternalClusterConfiguration(
            new Uri(url),
            Environment.GetEnvironmentVariable("MY_DEV_CLUSTER_KEY")
        );
    }
}

Changes

• New ExternalClusterConfiguration class (URI + optional API key + connectivity
  validation via GET /)
• ElasticsearchCluster<TConfiguration>:
  • TryUseExternalCluster() — virtual hook, returns null by default
  • TEST_ELASTICSEARCH_URL / TEST_ELASTICSEARCH_API_KEY env var detection
  • NodesUris() override returns external URI when in external mode
  • DisposeAsync() is a no-op for external clusters
  • IsExternal / ExternalApiKey properties for inspection
• ElasticsearchCluster (non-generic): default Client auto-wires API key auth
• Complex example: added ProgrammaticExternalCluster demo, API key handling
  in TestGenericCluster
• Updated READMEs for both Core and Elasticsearch packages

Co-authored-by: Cursor [email protected]