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/Utils/arm/request.ts

@@ -9,6 +9,7 @@ import promiseRetry, { AbortError } from "p-retry";
 import { HttpHeaders } from "../../Common/Constants";
 import { configContext } from "../../ConfigContext";
 import { userContext } from "../../UserContext";
+import { fetchWithTimeout } from "../FetchWithTimeout";
 
 interface ErrorResponse {
   code: string;
@@ -79,12 +80,14 @@ export async function armRequestWithoutPolling<T>({
     ...(customHeaders || {}),
   };
 
-  const response = await window.fetch(url.href, {
+  const fetchInit: RequestInit = {
     method,
     headers,
     body: requestBody ? JSON.stringify(requestBody) : undefined,
-    signal,
-  });
+    ...(signal ? { signal } : {}),
+  };
+
+  const response = signal ? await window.fetch(url.href, fetchInit) : await fetchWithTimeout(url.href, fetchInit);
 
   if (!response.ok) {
     let error: ARMError;