Schema serialization is not determistic

[jordi-mas-dj]

Environment details

• OS type and version: Debian 11
• Python version: 3.13.3
• pip version: 25.1.1
• langchain-google-spanner version: 0.82

Steps to reproduce

See PR: #184

Problem

With the same graph model, the schema returned by get_schema from SpannerGraphStore has a different order each time and it is not deterministic.

Inconsistent serialization undermines debugging, evaluation, and reproducibility.

The root cause is that the sets used to store do not preserve the order of the keys.