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/Divi/Schema/DiviCatalogAbilityTest.php

@@ -78,10 +78,10 @@ final class DiviCatalogAbilityTest extends TestCase
 
     public function testUnknownModuleAndPermission(): void
     {
-        self::assertSame('lit_not_found', $this->ability()->execute(['module' => 'slider-pro'])->get_error_code());
+        self::assertToolError($this->ability()->execute(['module' => 'slider-pro']), 'not_found');
 
         Functions\when('current_user_can')->justReturn(false);
-        self::assertSame('lit_forbidden', $this->ability()->permission([])->get_error_code());
+        self::assertToolError($this->ability()->permission([]), 'forbidden');
         self::assertTrue($this->ability()->annotations()['readonly']);
         self::assertTrue($this->ability()->isDirect());
     }