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/Pages/DiviWriterTest.php

@@ -54,9 +54,9 @@ final class DiviWriterTest extends TestCase
         $writer = $this->writer(Mockery::mock(DiviSaver::class), FakeChangeLog::forbidden());
 
         self::assertInstanceOf(DiviPage::class, $writer->page(['post_id' => 7, 'tree_rev' => Rev::of($content)]));
-        self::assertSame('lit_rev_mismatch', $writer->page(['post_id' => 7])->get_error_code());
+        self::assertToolError($writer->page(['post_id' => 7]), 'rev_mismatch');
         $stale = $writer->page(['post_id' => 7, 'tree_rev' => Rev::ofPost(DiviFixtures::givenPost($content))]);
-        self::assertSame('lit_rev_mismatch', $stale->get_error_code(), 'the post rev is no tree_rev');
+        self::assertToolError($stale, 'rev_mismatch', 'the post rev is no tree_rev');
         self::assertStringContainsString('nichts wurde', $stale->get_error_message());
     }
 
@@ -72,7 +72,7 @@ final class DiviWriterTest extends TestCase
             Mockery::mock(DiviComposition::class)
         );
 
-        self::assertSame('lit_unsupported', $writer->page(['post_id' => 7, 'tree_rev' => Rev::of($content)])->get_error_code());
+        self::assertToolError($writer->page(['post_id' => 7, 'tree_rev' => Rev::of($content)]), 'unsupported');
     }
 
     public function testSectionLabelOfANode(): void