test: ✅ Add regression test for contract-only Expression in where(). (#593)

Locks in the fix from PR #590, which switched src/CacheKey.php to import
the Illuminate\Contracts\Database\Query\Expression interface instead of
the concrete Illuminate\Database\Query\Expression class. Without the fix,
custom Expression classes that implement the contract but do not extend
the concrete class (e.g. tpetry/laravel-postgresql-enhanced) fall through
CacheKey's instanceof checks and trigger an "Object could not be converted
to string" fatal at src/CacheKey.php:280 in getOtherClauses().

The new test fixture (ContractOnlyExpression) implements only the contract,
and the integration test passes it as the column argument to where() to
exercise the formerly-broken code path.

Author: mikebronner

tests/Fixtures/ContractOnlyExpression.php

@@ -0,0 +1,20 @@
+<?php
+
+declare(strict_types=1);
+
+namespace GeneaLabs\LaravelModelCaching\Tests\Fixtures;
+
+use Illuminate\Contracts\Database\Query\Expression as ExpressionContract;
+use Illuminate\Database\Grammar;
+
+final class ContractOnlyExpression implements ExpressionContract
+{
+    public function __construct(private readonly string $value)
+    {
+    }
+
+    public function getValue(Grammar $grammar): string
+    {
+        return $this->value;
+    }
+}

tests/Integration/CachedBuilder/ContractExpressionTest.php

@@ -0,0 +1,35 @@
+<?php
+
+namespace GeneaLabs\LaravelModelCaching\Tests\Integration\CachedBuilder;
+
+use GeneaLabs\LaravelModelCaching\Tests\Fixtures\Book;
+use GeneaLabs\LaravelModelCaching\Tests\Fixtures\ContractOnlyExpression;
+use GeneaLabs\LaravelModelCaching\Tests\Fixtures\UncachedBook;
+use GeneaLabs\LaravelModelCaching\Tests\IntegrationTestCase;
+
+class ContractExpressionTest extends IntegrationTestCase
+{
+    /**
+     * Regression for PR #590. A custom Expression that implements
+     * Illuminate\Contracts\Database\Query\Expression but does NOT extend
+     * the concrete Illuminate\Database\Query\Expression must still be
+     * recognized by CacheKey's instanceof checks. Otherwise the object
+     * falls through into string concatenation in getOtherClauses() and
+     * triggers a fatal "Object of class … could not be converted to string".
+     */
+    public function test_cache_key_handles_contract_only_expression_as_where_column()
+    {
+        $expression = new ContractOnlyExpression('id');
+
+        $cached = (new Book)
+            ->where($expression, '>', 0)
+            ->get();
+
+        $live = (new UncachedBook)
+            ->where($expression, '>', 0)
+            ->get();
+
+        $this->assertNotEmpty($cached);
+        $this->assertEmpty($cached->diffKeys($live));
+    }
+}