fix: resolve memory leaks from missing IDisposable implementations and undisposed resources

[Copilot]

Audit of disposable resource handling found 6 leak sites — undisposed SemaphoreSlim, DbConnection, and Azure ServiceBus resources.

Critical

• DbDataReaderAsyncEnumerator2 — DbConnection from factory was assigned to a local in MoveNextAsync() and never disposed. Stored as field; disposed in DisposeAsync(). Compare with DbDataReaderAsyncEnumerator which already did this correctly.

// Before: connection leaked
var dbConnection = await _dbConnectionFactory(_cancellationToken);

// After: stored and disposed
_dbConnection = await _dbConnectionFactory(_cancellationToken);
// ... in DisposeAsync():
if (_dbConnection != null)
    await _dbConnection.DisposeAsync().ConfigureAwait(false);

• AzureServiceBusQueue — held SemaphoreSlim, ServiceBusClient, ServiceBusSender, ServiceBusReceiver with no IDisposable/IAsyncDisposable. Implemented both.

• AzureEventHubReceiverQueue — held SemaphoreSlim + EventHubClient with no IDisposable. Implemented.

Medium

• KafkaReceiverQueue — _consumerStartSemaphore.Dispose() missing from Dispose(bool). Sibling classes (KafkaEventStreamConsumer, KafkaSchemaRegistryReceiverQueue) already had this.

• Memory.SortedSet — SemaphoreSlim field, no IDisposable. Implemented.

• Sql.Mapping.Table — SemaphoreSlim field, no IDisposable. Implemented.

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