fix: replace +00:00 with Z

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: AmaseCocoa

src/apmodel/extra/cid/data_integrity_proof.py

@@ -36,6 +36,6 @@ def to_json(self):
 
         # Apply DataIntegrityProof-specific serialization for 'created' field
         if "created" in data and isinstance(data["created"], datetime):
-            data["created"] = data["created"].isoformat(timespec='seconds') + 'Z' # Convert datetime to ISO string with Z
+            data["created"] = data["created"].isoformat(timespec='seconds').replace('+00:00', 'Z') # Convert datetime to ISO string with Z
 
         return data