Implement ResetStaticFields method for Unity instance cleanup

[ImDreamerDev]

This PR adds support for Unity projects that have Domain Reloading disabled in Enter Play Mode Options.

Normally, Unity resets static fields when entering Play Mode, but when Domain Reloading is disabled, static data can remain from previous play sessions. This can cause unexpected behavior, such as old singleton instances, cached data, or test states carrying over.

To prevent this, static reset methods have been added to a few core SDK classes. These methods are called automatically by Unity when the subsystem is initialized, ensuring the SDK starts from a clean state each time.

Changes made:

• Added a reset method to Log to clear the current logger instance.
• Added a reset method to SpacetimeDBNetworkManager to clear the singleton instance.
• Added a reset method to RemoteTableHandleBase to clear the cached serializer instance.
• Added a reset method to DbConnectionBase to reset the testing flag.
• Added the required Unity imports for using RuntimeInitializeOnLoadMethod.

These changes should make the SDK more reliable when used in Unity projects that disable Domain Reloading, without affecting normal Unity projects that keep it enabled.

API and ABI breaking changes

No API or ABI breaking changes.

This only changes internal static state handling and does not modify existing public APIs or behavior.

Expected complexity level and risk

Complexity: 2/5

This is a relatively small change focused on improving Unity compatibility.

The main consideration is making sure the reset methods run at the correct time during Unity initialization and that clearing static fields does not interfere with normal SDK usage. The changes are isolated to a few classes that manage static state.

Testing

Tested the changes in Unity with Domain Reloading disabled to confirm that static data is properly reset between play sessions.

Verified that:

• The SDK initializes correctly after entering Play Mode multiple times.

• Singleton instances do not persist unexpectedly.

• Cached serializers and test state are cleared correctly.

• Existing behavior remains unchanged when Domain Reloading is enabled.

• Test normal Unity Play Mode behavior with Domain Reloading enabled.

• Test with Unity Enter Play Mode Options with Domain Reloading disabled.

[CLAassistant]

All committers have signed the CLA.

[rekhoff]

Thank you for your contribution! A few things I noted when looking over your changes.