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/Abilities/CursorTest.php

@@ -46,7 +46,7 @@ final class CursorTest extends TestCase
 
     public function testInvalidCursorErrorIsAValidationError(): void
     {
-        self::assertSame('lit_validation_failed', Cursor::invalid()->get_error_code());
+        self::assertToolError(Cursor::invalid(), 'validation_failed');
     }
 
     public function testInputPropertiesDescribeLimitAndCursor(): void