Report Unused using diagnostics as hint instead of warning (#8997)

Fix the issue that unused-using Diagnostics are reported as warning
instead of hint when there are linters defined in tspconfig.yaml

fixes #8692

Author: RodgeFu

.chronus/changes/unused-as-hint-2025-10-13-17-12-1.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/compiler"
+---
+
+UnusedUsing Diagnostics are reported as warning instead of hint when there are linters defined in tspconfig.yaml

packages/compiler/src/server/compile-service.ts

@@ -15,7 +15,7 @@ import { CompilerOptions } from "../core/options.js";
 import { parse } from "../core/parser.js";
 import { getBaseFileName, getDirectoryPath } from "../core/path-utils.js";
 import type { CompilerHost, TypeSpecScriptNode } from "../core/types.js";
-import { distinctArray } from "../utils/misc.js";
+import { deepClone, distinctArray } from "../utils/misc.js";
 import { getLocationInYamlScript } from "../yaml/diagnostics.js";
 import { parseYaml } from "../yaml/parser.js";
 import { ClientConfigProvider } from "./client-config-provider.js";
@@ -141,8 +141,10 @@ export function createCompileService({
     const [optionsFromConfig, _] = resolveOptionsFromConfig(config, {
       cwd: getDirectoryPath(path),
     });
+    // we need to keep the optionsFromConfig unchanged which will be returned in CompileResult
+    const clone = deepClone(optionsFromConfig);
     const options: CompilerOptions = {
-      ...optionsFromConfig,
+      ...clone,
       ...serverOptions,
       ...(additionalOptions ?? {}),
     };