Fix case-sensitivity bug in Cosmos DB queries for GetMessageCountAsync and ClearMessagesAsync

The count and delete queries used c.Type (capital T) instead of c.type (lowercase t),
which doesn't match the stored document property name. Cosmos DB property names are
case-sensitive, so these queries would always return 0 results.

Fixes the same issue as microsoft#3485 but applied cleanly to current main.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: alliscode

dotnet/src/Microsoft.Agents.AI.CosmosNoSql/CosmosChatHistoryProvider.cs

@@ -435,7 +435,7 @@ public async Task<int> GetMessageCountAsync(AgentSession? session, CancellationT
         var partitionKey = BuildPartitionKey(state);
 
         // Efficient count query
-        var query = new QueryDefinition("SELECT VALUE COUNT(1) FROM c WHERE c.conversationId = @conversationId AND c.Type = @type")
+        var query = new QueryDefinition("SELECT VALUE COUNT(1) FROM c WHERE c.conversationId = @conversationId AND c.type = @type")
             .WithParameter("@conversationId", state.ConversationId)
             .WithParameter("@type", "ChatMessage");
 
@@ -469,7 +469,7 @@ public async Task<int> ClearMessagesAsync(AgentSession? session, CancellationTok
         var partitionKey = BuildPartitionKey(state);
 
         // Batch delete for efficiency
-        var query = new QueryDefinition("SELECT VALUE c.id FROM c WHERE c.conversationId = @conversationId AND c.Type = @type")
+        var query = new QueryDefinition("SELECT VALUE c.id FROM c WHERE c.conversationId = @conversationId AND c.type = @type")
             .WithParameter("@conversationId", state.ConversationId)
             .WithParameter("@type", "ChatMessage");