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/Media/MediaSearchAbilityTest.php

@@ -68,7 +68,7 @@ final class MediaSearchAbilityTest extends TestCase
         $result = (new MediaSearchAbility($this->presenter(), Mockery::mock(MediaUsageFinder::class)))
             ->execute(['missing_alt' => true, 'mime' => 'application/pdf']);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
     }
 
     public function testMissingAltAllowsAnImageSubtypeMime(): void
@@ -108,7 +108,7 @@ final class MediaSearchAbilityTest extends TestCase
         $result = (new MediaSearchAbility($this->presenter(), Mockery::mock(MediaUsageFinder::class)))
             ->execute(['cursor' => '???']);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
     }
 
     public function testExcludesAttachmentsTheUserCannotEdit(): void
@@ -303,7 +303,7 @@ final class MediaSearchAbilityTest extends TestCase
         Functions\when('current_user_can')->justReturn(false);
         $ability = new MediaSearchAbility($this->presenter(), Mockery::mock(MediaUsageFinder::class));
 
-        self::assertSame('lit_forbidden', $ability->permission([])->get_error_code());
+        self::assertToolError($ability->permission([]), 'forbidden');
         self::assertTrue($ability->isDirect());
     }
 }