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/UserContext.ts

@@ -1,5 +1,4 @@
 import { CosmosDbArtifactType, ResourceTokenInfo } from "Contracts/FabricMessagesContract";
-import { ParsedResourceTokenConnectionString } from "Platform/Hosted/Helpers/ResourceTokenUtils";
 import { Action } from "Shared/Telemetry/TelemetryConstants";
 import { traceOpen } from "Shared/Telemetry/TelemetryProcessor";
 import { useCarousel } from "hooks/useCarousel";
@@ -111,7 +110,6 @@ export interface UserContext {
   readonly postgresConnectionStrParams?: PostgresConnectionStrParams;
   readonly isReplica?: boolean;
   collectionCreationDefaults: CollectionCreationDefaults;
-  sampleDataConnectionInfo?: ParsedResourceTokenConnectionString;
   readonly selectedRegionalEndpoint?: string;
   readonly writeEnabledInSelectedRegion?: boolean;
   readonly vcoreMongoConnectionParams?: VCoreMongoConnectionParams;