Server-side batching (SSB)

[bevzzz]

This PR adds BatchContext which implements Server-Side Batching protocol.

List<WeaviateObject<Map<String, Object>>> objects = List.of(...);
List<ObjectReference> references = List.of();
List<CompletableFuture<TaskHandle>> submitted = new ArrayList<>();

try (BatchContext<?> batch = collection.batch.start()) {
  
  // Optional: Define a retry policy that retries each task up to three times.
  Function<TaskHandle, CompletableFuture<TaskHandle>> retryMax3Times = task -> task
    .result().thenCompose(result -> {
      if (result.error().isEmpty() {
        return result;
      }
      if (task.timesRetried == 3) {
        throw new RuntimeException(result.error().get());
      }
      return retryMax3Times.apply(batch.retry(task));
    });

  // Submit all objects to the batch
  objects.map(batch::add).map(retryMax3Times).forEach(submitted::add);
  
  // Submit all references to the batch
  references.map(batch::add).map(retryMax3Times).forEach(submitted::add;
}

// Await until all tasks complete
CompletableFuture.allOf(submitted).get();

Description

See BatchContext Javadoc for a detailed architecture discussion.

Testing

BatchContext does all the heavy lifting: state management, thread coordination, etc.
To validate different scenario combinations (server shutdown, backoff, batch overflow, stream hangup, OOM) BatchContextTest uses a simple harness that lets it emit server-side events and await client-side messages.

BatchITest integration test helps verify that the client produces valid messages and adheres to to SSB protocol.

Miscellaneous

• ConsistencyLevel and Tenant are not Optional type in collection handle defaults to make their "optionality" more explicit.
• Merged VectorizersITest with SearchITest as both share the same setup, which takes CI time to spin up twice
• Deleted the now-redundant file_replication.proto and the associated generated protobuf stubs.
• Separated unit tests into their own CI job. There's no reason to run them for each Weaviate version, as they never hit the server. Plus it lets us catch smaller errors before starting the heavy work of spinning up containers.

[orca-security-eu]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   0   0   0	View in Orca
Passed	SAST	0   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca