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/MediaGetAbilityTest.php

@@ -45,7 +45,7 @@ final class MediaGetAbilityTest extends TestCase
         Functions\when('get_post')->justReturn(new WP_Post(['ID' => 5, 'post_type' => 'page']));
         $ability = new MediaGetAbility(Mockery::mock(MediaPresenter::class), Mockery::mock(MediaUsageFinder::class));
 
-        self::assertSame('lit_not_found', $ability->permission(['id' => 5])->get_error_code());
+        self::assertToolError($ability->permission(['id' => 5]), 'not_found');
     }
 
     public function testIsIndirectAndReadonly(): void