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/AuthorizationUtils.test.ts

@@ -38,10 +38,6 @@ describe("AuthorizationUtils", () => {
         ttl90Days: false,
         enableThroughputCap: false,
         enableHierarchicalKeys: false,
-        enableCopilot: false,
-        disableCopilotPhoenixGateaway: false,
-        enableCopilotFullSchema: false,
-        copilotChatFixedMonacoEditorHeight: false,
         enablePriorityBasedExecution: false,
         disableConnectionStringLogin: false,
         enableCloudShell: false,