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/QueryCopilot/Popup/DeletePopup.tsx

@@ -1,49 +0,0 @@
-import { DefaultButton, Modal, PrimaryButton, Stack, Text } from "@fluentui/react";
-import React, { Dispatch, SetStateAction } from "react";
-
-export const DeletePopup = ({
-  showDeletePopup,
-  setShowDeletePopup,
-  setQuery,
-  clearFeedback,
-  showFeedbackBar,
-}: {
-  showDeletePopup: boolean;
-  setShowDeletePopup: Dispatch<SetStateAction<boolean>>;
-  setQuery: Dispatch<SetStateAction<string>>;
-  clearFeedback: Dispatch<SetStateAction<void>>;
-  showFeedbackBar: Dispatch<SetStateAction<boolean>>;
-}): JSX.Element => {
-  const deleteCode = () => {
-    setQuery("");
-    setShowDeletePopup(false);
-    clearFeedback();
-    showFeedbackBar(false);
-  };
-
-  return (
-    <Modal
-      isOpen={showDeletePopup}
-      styles={{ main: { minHeight: "122px", minWidth: "880px" } }}
-      titleAriaId="deleteDialogTitle"
-      subtitleAriaId="deleteDialogSubTitle"
-    >
-      <Stack style={{ padding: "16px 24px", height: "auto" }}>
-        <Text id="deleteDialogTitle" style={{ height: 24, fontSize: "18px" }}>
-          <b>Delete code?</b>
-        </Text>
-        <Text id="deleteDialogSubTitle" style={{ marginTop: 10, marginBottom: 20 }}>
-          This will clear the query from the query builder pane along with all comments and also reset the prompt pane
-        </Text>
-        <Stack horizontal tokens={{ childrenGap: 10 }} horizontalAlign="start">
-          <PrimaryButton style={{ padding: "0px 20px", height: 24 }} onClick={deleteCode}>
-            Delete
-          </PrimaryButton>
-          <DefaultButton style={{ padding: "0px 20px", height: 24 }} onClick={() => setShowDeletePopup(false)}>
-            Close
-          </DefaultButton>
-        </Stack>
-      </Stack>
-    </Modal>
-  );
-};