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/DiviSaverTest.php

@@ -171,7 +171,7 @@ final class DiviSaverTest extends TestCase
             self::fail('WriteFailed expected');
         } catch (WriteFailed $e) {
             self::assertFalse($e->rolledBack);
-            self::assertSame('lit_partial_write', $e->toError()->get_error_code());
+            self::assertToolError($e->toError(), 'partial_write');
         }
     }