Guard expression resolver against malformed-UTF-8 json_encode

resolveVariable() declares a string return type, but json_encode returns
false on malformed UTF-8 (plausible for scraped feed/HTTP payloads),
which throws a TypeError under strict_types and fails the node. Encode
with JSON_PARTIAL_OUTPUT_ON_ERROR and fall back to an empty string.

app/Services/Automation/ExpressionResolver.php

@@ -58,6 +58,8 @@ private function resolveVariable(string $path, array $context): string
             return (string) $value;
         }
 
-        return json_encode($value);
+        // json_encode returns false on malformed UTF-8 (plausible for scraped
+        // feed/HTTP payloads); the method must still return a string.
+        return json_encode($value, JSON_PARTIAL_OUTPUT_ON_ERROR) ?: '';
     }
 }

tests/Unit/Automation/ExpressionResolverTest.php

@@ -50,3 +50,17 @@
 it('passes through templates with no variables', function () {
     expect($this->resolver->resolve('plain text', []))->toBe('plain text');
 });
+
+it('json-encodes array values', function () {
+    $context = ['fetched' => ['tags' => ['a', 'b']]];
+
+    expect($this->resolver->resolve('{{ fetched.tags }}', $context))->toBe('["a","b"]');
+});
+
+it('does not throw when an array value contains malformed UTF-8', function () {
+    // Scraped feed/HTTP payloads can carry invalid UTF-8; json_encode returns
+    // false for it, but the resolver must still yield a string.
+    $context = ['fetched' => ['body' => ["bad\xB1utf8"]]];
+
+    expect($this->resolver->resolve('{{ fetched.body }}', $context))->toBeString();
+});