fix: make BusProNet response failures diagnosable and non-fatal

src/BusProNet/ResponseParser.php

@@ -30,10 +30,7 @@ class ResponseParser
      */
     public function parseXmlString(string $type, string $content): mixed
     {
-        $xml = simplexml_load_string($content);
-        if (false === $xml) {
-            throw new ResponseParserException('Unable to parse XML response');
-        }
+        $xml = $this->loadXml($content);
 
         // Override type when present in XML to catch error responses
         $responseType = $type;
@@ -65,7 +62,7 @@ class ResponseParser
                 return $this->createHotelsResponse($xml);
         }
 
-        throw new ResponseParserException('Unable to parse XML response');
+        throw new ResponseParserException(sprintf('Unrecognised BusProNet response type "%s"', $responseType));
     }
 
     public function createNotificationResponse(\SimpleXMLElement $xml): NotificationResponse
@@ -302,6 +299,50 @@ class ResponseParser
         return new BaseDataResponse($hotels);
     }
 
+    /**
+     * The BusProNet endpoint can answer with an empty or truncated body. Keep the libxml
+     * reason instead of collapsing every shape of broken response into one message.
+     *
+     * @throws ResponseParserException
+     */
+    private function loadXml(string $content): \SimpleXMLElement
+    {
+        if ('' === trim($content)) {
+            throw new ResponseParserException(sprintf('Unable to parse XML response (%d bytes): empty response', strlen($content)));
+        }
+
+        $previousUseErrors = libxml_use_internal_errors(true);
+        libxml_clear_errors();
+
+        try {
+            $xml = simplexml_load_string($content);
+
+            if (false === $xml) {
+                throw new ResponseParserException(sprintf('Unable to parse XML response (%d bytes): %s', strlen($content), $this->describeLibxmlErrors()));
+            }
+
+            return $xml;
+        } finally {
+            libxml_clear_errors();
+            libxml_use_internal_errors($previousUseErrors);
+        }
+    }
+
+    private function describeLibxmlErrors(): string
+    {
+        $messages = [];
+
+        foreach (libxml_get_errors() as $error) {
+            $messages[] = sprintf('%s (line %d, column %d)', trim($error->message), $error->line, $error->column);
+        }
+
+        if ([] === $messages) {
+            return 'unknown XML error';
+        }
+
+        return implode('; ', array_unique($messages));
+    }
+
     private function resolveOptions(array $options): array
     {
         $optionsResolver = new OptionsResolver();