Fix build warnings in jnienv-gen project

[simonrozsival]

This change removes the following build warning:

jnienv-gen net10.0 succeeded with 1 warning(s) (3.1s) → bin/BuildDebug/jnienv-gen.dll
    /.../build-tools/jnienv-gen/Generator.cs(524,15): warning CS0649: Field 'JniFunction.Prebind' is never assigned to, and will always have its default value false

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Pull Request
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• Useful description of why the change is necessary.
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[Copilot]

Pull request overview

This PR removes a previously grandfathered warning exemption for the jnienv-gen project by eliminating the underlying unused Prebind field and its associated dead code paths in the generator, so the project can build cleanly with warnings treated as errors on CI.

Changes:

• Remove the jnienv-gen.csproj carve-out from _AllowProjectWarnings, allowing CI’s TreatWarningsAsErrors behavior to apply.
• Delete the unused JniFunction.Prebind field and simplify GenerateJniNativeInterfaceInvoker to always emit the lazy-initialization property form.

Show a summary per file

File	Description
Directory.Build.props	Removes the jnienv-gen warning allowance so CI treats warnings as errors for the project.
build-tools/jnienv-gen/Generator.cs	Removes the unused Prebind flag and simplifies generated invoker delegate binding code accordingly.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 0