test: AssertsToolErrors helper for tool error assertions (T3)

tests/Support/AssertsToolErrors (used by TestCase): assertToolError($result, code)
checks the WP_Error code lit_<code> and the code of the client JSON and returns the
payload; clientError() decodes the payload; errorFields() returns the fields and
asserts they exist. Replaces the private clientError() decoders of
DiviWriteAbilitiesTest/DiviReplaceTextAbilityTest, 216 single-line
assertSame('lit_…', …->get_error_code()) assertions, 24 get_error_data()['fields']
digs and two json_decode(message) reads. ErrorsTest keeps json_decode: it tests
the format itself. Tests 1404 -> 1409 (AssertsToolErrorsTest); no test has fewer
assertions.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tests/Unit/Integrations/WordPress/Menus/MenuCreateAbilityTest.php

@@ -20,7 +20,7 @@ final class MenuCreateAbilityTest extends TestCase
         $ability = new MenuCreateAbility(FakeChangeLog::forbidden());
         $result = $ability->execute(['name' => 'Neu', 'location' => 'nope']);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
         self::assertStringContainsString('primary-menu', $result->get_error_message());
     }
 
@@ -32,7 +32,7 @@ final class MenuCreateAbilityTest extends TestCase
 
         $result = (new MenuCreateAbility(FakeChangeLog::forbidden()))->execute(['name' => 'Hauptmenü']);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
     }
 
     public function testCreatesAndAssignsTheLocation(): void
@@ -65,6 +65,6 @@ final class MenuCreateAbilityTest extends TestCase
 
         $ability = new MenuCreateAbility(FakeChangeLog::forbidden());
 
-        self::assertSame('lit_forbidden', $ability->permission(['name' => 'Neu'])->get_error_code());
+        self::assertToolError($ability->permission(['name' => 'Neu']), 'forbidden');
     }
 }