Fix GPLearn research example code errors (#2162)

[AlexCatarino]

Summary

• Remove undefined `predict()` call in the Research Environment GPLearn example (was never defined; predictions were already computed as `y_predict` earlier in the snippet)
• Add missing `qb.object_store.save()` calls to persist the transformer and regressor models to the object store

Resolves #2162

Backtest results (Writing Algorithms example, Legacy env, 2024-09-01–2024-12-31)

Metric	Value
Sharpe Ratio	-1.036
Total Orders	23
Net Profit	-2.635%
Compounding Annual Return	-7.695%
Win Rate	47%
Max Drawdown	9.0%
Probabilistic Sharpe Ratio	15.0%

Project: https://www.quantconnect.com/project/29164258 (leanEnvironment=7, Legacy)

Test plan

• Writing Algorithms GPLearn example runs without errors in the Legacy package environment (leanEnvironment=7)
• Verify the page renders correctly on quantconnect.com/docs
• Confirm the Research Environment GPLearn notebook example runs without errors in the Legacy package environment
• Check all links resolve