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/Support/AssertsToolErrors.php

@@ -0,0 +1,47 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Tests\Support;
+
+use PHPUnit\Framework\Assert;
+use WP_Error;
+
+/**
+ * Assertions on the errors abilities return (Errors::make(): WP_Error code "lit_<code>", the
+ * message carries the client JSON {code, message, hint, fields?, …}).
+ */
+trait AssertsToolErrors
+{
+    /**
+     * Asserts a tool error with $code as the client sees it (without "lit_"): the WP_Error code
+     * and the code in the JSON payload.
+     *
+     * @return array<string, mixed> The client payload.
+     */
+    public static function assertToolError(mixed $result, string $code, string $message = ''): array
+    {
+        Assert::assertInstanceOf(WP_Error::class, $result, $message);
+        Assert::assertSame('lit_' . $code, $result->get_error_code(), $message);
+        $payload = self::clientError($result);
+        Assert::assertSame($code, $payload['code'] ?? null, $message);
+
+        return $payload;
+    }
+
+    /** @return array<string, mixed> The structured error the client sees (message JSON, spec §8.2). */
+    public static function clientError(WP_Error $error): array
+    {
+        return (array) json_decode($error->get_error_message(), true);
+    }
+
+    /** @return array<string, string> The error's fields (input path => problem); asserts it has some. */
+    public static function errorFields(WP_Error $error): array
+    {
+        $data = $error->get_error_data();
+        Assert::assertIsArray($data, 'error data');
+        Assert::assertIsArray($data['fields'] ?? null, 'error fields');
+
+        return $data['fields'];
+    }
+}