fix: preserve full error message in not() expectation failures

When using not() expectations with custom error messages, the message
was truncated because throwExpectationFailedException() passed all
arguments through shortenedExport() which limits strings to ~40 chars.

Uses the full export() method for arguments instead of shortenedExport()
so custom error messages are displayed in their entirety.

Fixes #1533

src/Support/Exporter.php

@@ -86,4 +86,17 @@ final readonly class Exporter
 
         return (string) preg_replace(array_keys($map), array_values($map), $this->exporter->shortenedExport($value));
     }
+
+    /**
+     * Exports a value into a full single-line string without truncation.
+     */
+    public function export(mixed $value): string
+    {
+        $map = [
+            '#\\\n\s*#' => '',
+            '# Object \(\.{3}\)#' => '',
+        ];
+
+        return (string) preg_replace(array_keys($map), array_values($map), $this->exporter->export($value));
+    }
 }