build(benchmark): upgrade to ESLint 9 with flat config (microsoft#26412)

## Summary
- Replace legacy `.eslintrc.cjs` with ESLint 9 `eslint.config.mts` flat
config
- Remove `@fluidframework/eslint-config-fluid` (not ESLint 9 compatible
yet); added TODO to re-enable once v9 is released
- Add required plugins directly: `typescript-eslint`,
`eslint-plugin-import-x`, `eslint-plugin-unicorn`,
`eslint-config-prettier`, `eslint-import-resolver-typescript`, and
`jiti`
- Upgrade `eslint` from `~8.57.0` to `~9.39.1`
- Update `eslint-disable` comments from `import/` to `import-x/` prefix

Author: tylerbutler

tools/benchmark/.eslintrc.cjs

@@ -0,0 +1,88 @@
+/*!
+ * Copyright (c) Microsoft Corporation and contributors. All rights reserved.
+ * Licensed under the MIT License.
+ */
+
+// TODO: Replace this local config with the shared @fluidframework/eslint-config-fluid
+// flat config once v9 is released. Example:
+//   import { recommended } from "@fluidframework/eslint-config-fluid/flat.mts";
+//   ...recommended,
+
+import eslint from "@eslint/js";
+import eslintConfigPrettier from "eslint-config-prettier";
+import importPlugin from "eslint-plugin-import-x";
+import unicorn from "eslint-plugin-unicorn";
+import type { Linter } from "eslint";
+import tseslint from "typescript-eslint";
+
+const config: Linter.Config[] = [
+	// Base ESLint recommended rules
+	eslint.configs.recommended,
+
+	// TypeScript ESLint recommended rules (type-checked)
+	...tseslint.configs.recommendedTypeChecked,
+
+	// import-x plugin for import rules
+	importPlugin.flatConfigs.recommended as Linter.Config,
+	importPlugin.flatConfigs.typescript as Linter.Config,
+
+	// Unicorn plugin - register plugin so eslint-disable comments for unicorn rules are valid.
+	// Only specific rules are enabled; the full recommended preset is not used due to a
+	// compatibility issue between eslint-plugin-unicorn@54 and eslint@9.39 (expiring-todo-comments).
+	{
+		plugins: {
+			unicorn,
+		},
+		rules: {
+			"unicorn/prefer-module": "error",
+			"unicorn/prefer-native-coercion-functions": "error",
+			"unicorn/prefer-ternary": "error",
+			"unicorn/no-negated-condition": "error",
+		},
+	},
+
+	// TypeScript parser and project configuration
+	{
+		languageOptions: {
+			parserOptions: {
+				projectService: true,
+				tsconfigRootDir: import.meta.dirname,
+			},
+		},
+		settings: {
+			"import-x/resolver": {
+				typescript: true,
+				node: true,
+			},
+		},
+	},
+
+	// Project-specific rule overrides
+	{
+		rules: {
+			"@typescript-eslint/no-shadow": "off",
+			"@typescript-eslint/no-unsafe-call": "off",
+			"@typescript-eslint/no-unsafe-member-access": "off",
+			"@typescript-eslint/require-await": "off",
+			"@typescript-eslint/restrict-template-expressions": "off",
+			"import-x/no-nodejs-modules": [
+				"error",
+				{ allow: ["node:v8", "perf_hooks", "node:child_process"] },
+			],
+		},
+	},
+
+	// Test file overrides
+	{
+		files: ["src/test/**"],
+		rules: {
+			"import-x/no-nodejs-modules": "off",
+		},
+	},
+
+	// Prettier config - disables rules that conflict with prettier. Must be last so
+	// no subsequent config objects can re-enable formatting rules.
+	eslintConfigPrettier,
+];
+
+export default config;

tools/benchmark/eslint.config.mts

@@ -45,7 +45,6 @@
 	"devDependencies": {
 		"@fluid-internal/mocha-test-setup": "~2.51.0",
 		"@fluidframework/build-common": "^2.0.3",
-		"@fluidframework/eslint-config-fluid": "^6.1.0",
 		"@microsoft/api-extractor": "^7.52.11",
 		"@types/chai": "^5.2.2",
 		"@types/mocha": "^10.0.10",
@@ -54,14 +53,22 @@
 		"concurrently": "^8.2.2",
 		"copyfiles": "^2.4.1",
 		"cross-env": "^7.0.3",
-		"eslint": "~8.57.0",
+		"eslint": "~9.39.1",
+		"eslint-config-prettier": "~10.1.8",
+		"eslint-import-resolver-typescript": "~4.4.4",
+		"eslint-plugin-import-x": "~4.16.1",
+		"eslint-plugin-unicorn": "~54.0.0",
+		"jiti": "^2.6.1",
 		"prettier": "~3.0.3",
 		"rimraf": "^4.4.1",
-		"typescript": "~5.4.5"
+		"typescript": "~5.4.5",
+		"typescript-eslint": "~8.54.0"
 	},
 	"packageManager": "pnpm@10.18.3+sha512.bbd16e6d7286fd7e01f6b3c0b3c932cda2965c06a908328f74663f10a9aea51f1129eea615134bf992831b009eabe167ecb7008b597f40ff9bc75946aadfb08d",
 	"pnpm": {
-		"onlyBuiltDependencies": ["unrs-resolver"],
+		"onlyBuiltDependencies": [
+			"unrs-resolver"
+		],
 		"overrides": {
 			"nanoid": "^3.3.9"
 		}