Fix ClientSim compatibility with SDK 3.6.1

[FxckingAngel]

Summary

• update UdonProductStub to implement the current IProduct.Purchase overload
• use string event names for ClientSim events that are not exposed as UdonManager constants in SDK 3.6.1

Validation

Tested in a clean world project using Unity 2022.3.22f1 and VRChat SDK 3.6.1.

Tundra build success (19.17 seconds), 13 items updated, 705 evaluated
Exiting without the bug reporter. Application will terminate with return code 0

[github-actions]

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[FxckingAngel]

Closing and reopening from a cleaner branch name.