docs(test): add fileoverview headers to utility test files

Add descriptive @fileoverview headers to utility function tests:
- lang.test.mts: null/undefined/empty string detection
- strings.test.mts: string validation, normalization, transformation
- helpers.test.mts: createHelpersNamespaceObject utility

Author: jdalton

test/helpers.test.mts

@@ -1,3 +1,8 @@
+/**
+ * @fileoverview Unit tests for helper utilities.
+ * Tests the createHelpersNamespaceObject function which reorganizes helper objects
+ * by property names, with support for defaults, custom comparators, and null prototypes.
+ */
 import { describe, expect, it } from 'vitest'
 
 import { createHelpersNamespaceObject } from '../src/helpers.js'

test/lang.test.mts

@@ -1,3 +1,7 @@
+/**
+ * @fileoverview Unit tests for language utility functions.
+ * Tests null/undefined/empty string detection utilities used throughout the codebase.
+ */
 import { describe, expect, it } from 'vitest'
 
 import { isNullishOrEmptyString } from '../src/lang.js'

test/strings.test.mts

@@ -1,3 +1,8 @@
+/**
+ * @fileoverview Unit tests for string manipulation utilities.
+ * Tests string validation, normalization, transformation, and comparison functions
+ * including whitespace detection, semver validation, case conversion, and character replacement.
+ */
 import { describe, expect, it } from 'vitest'
 
 import {