Add unit tests for AbstractType::convertMixedToString method

- Added tests to validate behavior of `convertMixedToString` for scalars, null, and `Stringable` objects.
- Confirmed exceptions are thrown for non-stringable objects, arrays, and resources.

Author: georgedeploy

tests/Unit/Abstract/AbstractTypeTest.php

@@ -0,0 +1,45 @@
+<?php
+
+declare(strict_types=1);
+
+use PhpTypedValues\Abstract\AbstractType;
+use PhpTypedValues\Exception\TypeException;
+
+it('convertMixedToString casts scalars and null to string', function (): void {
+    expect(AbstractType::convertMixedToString(123))->toBe('123')
+        ->and(AbstractType::convertMixedToString(1.5))->toBe('1.5')
+        ->and(AbstractType::convertMixedToString('foo'))->toBe('foo')
+        ->and(AbstractType::convertMixedToString(true))->toBe('1')
+        ->and(AbstractType::convertMixedToString(false))->toBe('')
+        ->and(AbstractType::convertMixedToString(null))->toBe('');
+});
+
+it('convertMixedToString accepts Stringable objects', function (): void {
+    $obj = new class implements Stringable {
+        public function __toString(): string
+        {
+            return '3.5';
+        }
+    };
+
+    expect(AbstractType::convertMixedToString($obj))->toBe('3.5');
+});
+
+it('convertMixedToString throws TypeException for non-stringable objects, arrays and resources', function (): void {
+    // array
+    expect(fn() => AbstractType::convertMixedToString(['x']))
+        ->toThrow(TypeException::class, 'Value cannot be cast to string');
+
+    // stdClass (non-stringable)
+    expect(fn() => AbstractType::convertMixedToString(new stdClass()))
+        ->toThrow(TypeException::class, 'Value cannot be cast to string');
+
+    // resource
+    $res = fopen('php://memory', 'r');
+    try {
+        expect(fn() => AbstractType::convertMixedToString($res))
+            ->toThrow(TypeException::class, 'Value cannot be cast to string');
+    } finally {
+        \is_resource($res) && fclose($res);
+    }
+});