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/TestCase.php

@@ -6,12 +6,14 @@ namespace LIT\MCP\Tests\Unit;
 
 use Brain\Monkey;
 use Brain\Monkey\Functions;
+use LIT\MCP\Tests\Support\AssertsToolErrors;
 use LIT\MCP\Tests\Support\FakeChangeLog;
 use Mockery\Adapter\Phpunit\MockeryPHPUnitIntegration;
 use PHPUnit\Framework\TestCase as BaseTestCase;
 
 abstract class TestCase extends BaseTestCase
 {
+    use AssertsToolErrors;
     use MockeryPHPUnitIntegration;
 
     protected function setUp(): void