Perf/copilot cleanup and optimizations (#2447)

* perf: remove deprecated copilot feature, add ARM timeouts, fix race conditions

- Remove entire QueryCopilot feature (~50 files deleted, ~30 files cleaned)
- Remove CopilotConfigured and SampleDataLoaded metric phases
- Fix DatabaseTreeRendered 76% stuck rate (remove one-shot guard in useMetricPhases)
- Add 8s default timeout to ARM requests (AbortController-based)
- Fix MSAL token forceRefresh (true -> false, use cache)
- Add concurrency limit of 5 to collection loading in Explorer
- Remove orphaned SampleDataClient.ts and queryCopilotSampleData.json
- Clean up dead sampleDataConnectionInfo field from UserContext

* Clean up copilot and optimize initialization

* Clean up copilot and optimize initialization

src/Explorer/Tabs/QueryTab/ResultsView.tsx

@@ -42,7 +42,6 @@ import { logConsoleProgress } from "Utils/NotificationConsoleUtils";
 import create from "zustand";
 import { client } from "../../../Common/CosmosClient";
 import { handleError } from "../../../Common/ErrorHandlingUtils";
-import { sampleDataClient } from "../../../Common/SampleDataClient";
 import { ResultsViewProps } from "./QueryResultSection";
 import { useIndexAdvisorStyles } from "./StylesAdvisor";
 enum ResultsTabs {
@@ -583,8 +582,7 @@ export const IndexAdvisorTab: React.FC<{
         query: queryEditorContent,
       };
 
-      // Use sampleDataClient for CopilotSampleDB, regular client for other databases
-      const cosmosClient = databaseId === "CopilotSampleDB" ? sampleDataClient() : client();
+      const cosmosClient = client();
 
       const sdkResponse = await cosmosClient
         .database(databaseId)