fix unity-testsuite: clear intermediate Godot build state

[joshua-spacetime]

Description of Changes

Harden cargo regen csharp dlls against flaky Godot restore/build state in the Unity test suite.

I suspect this flake is runner specific. Some Unity runners appear to have incomplete Godot package state, where sdks/csharp/packages/godotsharp exists but is missing GodotSharp.dll. In that state, NuGet can still report restore success and generate a project.assets.json that contains GodotSharp metadata but no usable compile asset.

The regen flow now removes the Godot MSBuild intermediate directory (sdks/csharp/obj~/godot) before restoring SpacetimeDB.ClientSDK.Godot.csproj. It also clears the targeted GodotSharp package cache (sdks/csharp/packages/godotsharp) so NuGet cannot reuse a partially restored package missing GodotSharp.dll.

After restore, regen parses project.assets.json and verifies that GodotSharp is present and that the referenced GodotSharp.dll exists before running dotnet pack --no-restore.

This should either prevent the flaky missing-Godot namespace failure, or fail earlier with a clearer restore/package-state error.

NOTE/WARNING: This code was entirely AI generated.

API and ABI breaking changes

N/A

Expected complexity level and risk

~

Testing

• unity-testsuite passes (I ran it at least 5 times to try to get it scheduled to different runners)

[bfops]

Ship it