feat: structured errors with hint, ToolResult, cursor pagination

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01At3V6SjwbMe6eMzVGW3Hs7

tests/Unit/TestCase.php

@@ -5,6 +5,7 @@ declare(strict_types=1);
 namespace LIT\MCP\Tests\Unit;
 
 use Brain\Monkey;
+use Brain\Monkey\Functions;
 use Mockery\Adapter\Phpunit\MockeryPHPUnitIntegration;
 use PHPUnit\Framework\TestCase as BaseTestCase;
 
@@ -16,6 +17,18 @@ abstract class TestCase extends BaseTestCase
     {
         parent::setUp();
         Monkey\setUp();
+
+        // Pure helpers with obvious behaviour. Never use Functions\expect() on these
+        // names in a test: Brain Monkey forbids a stub and an expectation for the
+        // same function. Functions\when() may still redefine them.
+        Functions\stubs([
+            'wp_json_encode' => static fn ($data, int $flags = 0) => json_encode($data, $flags),
+            'is_wp_error' => static fn ($thing): bool => $thing instanceof \WP_Error,
+            'absint' => static fn ($value): int => abs((int) $value),
+            'wp_slash' => static fn ($value) => $value,
+            'wp_unslash' => static fn ($value) => $value,
+            'sanitize_text_field' => static fn ($value): string => trim(strip_tags((string) $value)),
+        ]);
     }
 
     protected function tearDown(): void