update id generator

Author: lusu-msft

sdk/agentserver/azure-ai-agentserver-core/azure/ai/agentserver/core/server/common/id_generator/foundry_id_generator.py

@@ -133,4 +133,4 @@ def _extract_partition_id(
         if len(segment) < string_length + partition_key_length:
             raise ValueError(f"Id '{id_str}' does not contain a valid id.")
 
-        return segment[-partition_key_length:]
+        return segment[:partition_key_length]