Add test for PDF module registration

Added a test case for `register_pdf` in `stdlib/src/pdf.rs` to ensure the module is registered properly with the expected metadata (name, version, empty capabilities and exports).

Co-authored-by: Tcode-Motion <188012755+Tcode-Motion@users.noreply.github.com>

Author: google-labs-jules[bot]

stdlib/src/pdf.rs

@@ -19,3 +19,21 @@ impl StdlibRegistry {
         );
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_register_pdf() {
+        let mut registry = StdlibRegistry::default();
+        registry.register_pdf();
+
+        assert!(registry.has_module("std.pdf"));
+        let module = registry.get_module("std.pdf").unwrap();
+        assert_eq!(module.name, "std.pdf");
+        assert_eq!(module.version, "1.0.0");
+        assert!(module.exports.is_empty());
+        assert!(module.required_capabilities.is_empty());
+    }
+}