fix: full-form internal links follow the {{link:}} target rules (ruling P10)

One PostIdParser for {{link:}} and select_post; post_link_url_<type> needs a
living, readable, non-attachment target of exactly that type.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tests/Unit/Integrations/Divi/Compose/PostIdParserTest.php

@@ -0,0 +1,30 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Tests\Unit\Integrations\Divi\Compose;
+
+use LIT\MCP\Integrations\Divi\Compose\PostIdParser;
+use PHPUnit\Framework\TestCase;
+
+final class PostIdParserTest extends TestCase
+{
+    public function testAcceptsPositiveIdsAsIntOrDigits(): void
+    {
+        self::assertSame(839, PostIdParser::parse(839));
+        self::assertSame(839, PostIdParser::parse('839'));
+        self::assertSame(PHP_INT_MAX, PostIdParser::parse((string) PHP_INT_MAX));
+    }
+
+    /** Ruling P10: one rule for {{link:}} and post_link_url_* – no zero, sign, padding, slug or overflow. */
+    public function testRefusesEverythingElse(): void
+    {
+        $bad = [
+            0, -3, '0', '-3', '+3', '0839', '8 39', ' 839', '839 ', "839\n", '839.0', '0x1f', '1e3', 'impressum',
+            '99999999999999999999', '9223372036854775808', 1.5, 839.0, null, [], true, false,
+        ];
+        foreach ($bad as $value) {
+            self::assertNull(PostIdParser::parse($value), var_export($value, true));
+        }
+    }
+}