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/DiviReadAbilitiesTest.php

@@ -65,7 +65,7 @@ final class DiviReadAbilitiesTest extends TestCase
         self::assertSame(['s1', 's1.r0'], array_column($result['nodes'], 'id'));
 
         $missing = $ability->execute(['post_id' => 7, 'id' => 's7']);
-        self::assertSame('lit_not_found', $missing->get_error_code());
+        self::assertToolError($missing, 'not_found');
         self::assertStringContainsString('tree_rev', $missing->get_error_message());
     }
 
@@ -120,7 +120,7 @@ final class DiviReadAbilitiesTest extends TestCase
         DiviFixtures::givenPost(DiviFixtures::markup('synthetic-basic.html'));
         $ability = new DiviNodeGetAbility(new DiviPageLoader(DiviFixtures::schema()), $this->design());
 
-        self::assertSame('lit_validation_failed', $ability->execute(['post_id' => 7, 'id' => 'hero'])->get_error_code());
-        self::assertSame('lit_not_found', $ability->execute(['post_id' => 7, 'id' => 's5.r0'])->get_error_code());
+        self::assertToolError($ability->execute(['post_id' => 7, 'id' => 'hero']), 'validation_failed');
+        self::assertToolError($ability->execute(['post_id' => 7, 'id' => 's5.r0']), 'not_found');
     }
 }