fix: Add topic cache invalidation and auto-creation on metadata requests

Add InvalidateTopicExistsCache method to SeaweedMQHandlerInterface and impl
ement cache refresh logic in metadata response handler.

When a consumer requests metadata for a topic that doesn't appear in the
cache (but was just created by a producer), force a fresh broker check
and auto-create the topic if needed with default partitions.

This fix attempts to address the consumer stalling issue by:
1. Invalidating stale cache entries before checking broker
2. Automatically creating topics on metadata requests (like Kafka's auto.create.topics.enable=true)
3. Returning topics to consumers more reliably

However, testing shows consumers still can't find topics even after creation,
suggesting a deeper issue with topic persistence or broker client communication.

Added InvalidateTopicExistsCache to mock handler as no-op for testing.

Note: Integration testing reveals that consumers get 'topic does not exist'
errors even when producers successfully create topics. This suggests the
real issue is either:
- Topics created by producers aren't visible to broker client queries
- Broker client TopicExists() doesn't work correctly
- There's a race condition in topic creation/registration

Requires further investigation of broker client implementation and SMQ
topic persistence logic.

test/kafka/kafka-client-loadtest/internal/consumer/consumer_stalling_test.go

@@ -107,8 +107,6 @@ func TestEmptyFetchShouldNotStopConsumer(t *testing.T) {
 		nextFetchOffset := committedOffset + 1
 		
 		// First attempt: get empty (transient - data might not be available yet)
-		firstFetchResult := 0 // bytes returned
-		
 		// WRONG behavior (bug): Consumer sees 0 bytes and stops
 		// wrongConsumerLogic := (firstFetchResult == 0)  // gives up!