feat(build-tools): support ESLint 9 flat config detection and parsing

[tylerbutler]

Description

Enables fluid-build to properly support ESLint 9 flat config files for incremental build tracking.

Changes

Config File Detection (taskUtils.ts)

Expands getEsLintConfigFilePath to detect ESLint 9 flat config files:

• eslint.config.{mjs,mts,cjs,cts,js,ts} (checked first)
• Legacy .eslintrc.{js,cjs,json} files (backwards compatibility)

Config File Parsing (fluidBuildTasks.ts)

Replaces manual config file parsing with ESLint's calculateConfigForFile() API to extract parserOptions.project for build task dependencies. This properly supports:

• ESLint 9 flat configs (languageOptions.parserOptions.project)
• Legacy eslintrc configs (parserOptions.project)
• TypeScript config files (.mts, .cts, .ts)
• ESM config files (.mjs)

The previous manual parsing with require() and JSON5.parse() couldn't handle ESLint 9's different config structure or TypeScript/ESM configs.

Motivation

Without these changes, packages using ESLint 9 flat configs cannot have proper incremental build tracking - fluid-build couldn't detect the config files or parse them to determine tsc task dependencies.

Related: #26017

[Copilot]

Pull request overview

This PR updates the getEsLintConfigFilePath function to detect ESLint 9 flat config files in addition to legacy .eslintrc files. This enables fluid-build to locate config files for packages migrating to ESLint 9, preventing errors during incremental build tracking.

Key Changes

• Added detection for ESLint 9 flat config files (.mjs, .mts, .cjs, .cts, .js, .ts variants)
• Maintained backwards compatibility with legacy eslintrc files
• Established precedence order with ESLint 9 configs checked first

Comments suppressed due to low confidence (1)

build-tools/packages/build-tools/src/fluidBuild/tasks/taskUtils.ts:41

• The function getEsLintConfigFilePath lacks test coverage. Other utility functions in the same file (like globFn and globWithGitignore) have comprehensive test coverage in src/test/globPatterns.test.ts.

Consider adding tests to verify:

• Correct detection of ESLint 9 flat config files in the proper precedence order
• Correct detection of legacy eslintrc files
• Behavior when multiple config files exist (should return the first match based on precedence)
• Behavior when no config file exists (returns undefined)

export function getEsLintConfigFilePath(dir: string) {
	// ESLint 9 flat config files (checked first as they take precedence)
	// Then legacy eslintrc files for backwards compatibility
	// TODO: we currently don't support .yaml and .yml, or config in package.json
	const possibleConfig = [
		// ESLint 9 flat config files
		"eslint.config.mjs",
		"eslint.config.mts",
		"eslint.config.cjs",
		"eslint.config.cts",
		"eslint.config.js",
		"eslint.config.ts",
		// Legacy eslintrc files
		".eslintrc.js",
		".eslintrc.cjs",
		".eslintrc.json",
		".eslintrc",
	];
	for (const configFile of possibleConfig) {
		const configFileFullPath = path.join(dir, configFile);
		if (existsSync(configFileFullPath)) {
			return configFileFullPath;
		}
	}
	return undefined;
}