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/Content/PostWriterTest.php

@@ -48,8 +48,8 @@ final class PostWriterTest extends TestCase
             'template' => 'fehlt.php',
         ], true);
 
-        $fields = $result->get_error_data()['fields'];
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        $fields = self::errorFields($result);
+        self::assertToolError($result, 'validation_failed');
         self::assertSame('Pflichtfeld fehlt.', $fields['fields.role']);
         self::assertSame('Taxonomie gehört nicht zu diesem Inhaltstyp.', $fields['terms.genre']);
         self::assertSame('Inhaltstyp unterstützt kein Beitragsbild.', $fields['featured_image']);
@@ -72,7 +72,7 @@ final class PostWriterTest extends TestCase
         $result = $writer->prepare('page', $input, false);
         self::assertFalse(is_array($result), $path . ' should be rejected');
 
-        return (string) $result->get_error_data()['fields'][$path];
+        return (string) self::errorFields($result)[$path];
     }
 
     public function testParentMustBeAReadableNonDescendantEntryOfAHierarchicalType(): void
@@ -152,7 +152,7 @@ final class PostWriterTest extends TestCase
             false
         );
 
-        self::assertSame(['fields.secret' => 'Keine Berechtigung, dieses Feld zu ändern.'], $result->get_error_data()['fields']);
+        self::assertSame(['fields.secret' => 'Keine Berechtigung, dieses Feld zu ändern.'], self::errorFields($result));
         self::assertSame([['edit_post_meta', 5, 'city'], ['edit_post_meta', 5, 'secret']], $checked);
     }