Fix analyzer DLL import settings before Unity refresh

[rafay-pk]

Summary

Fixes NuGet packages with Roslyn analyzer/source-generator DLLs, such as MemoryPack.Generator, being imported by Unity as normal plugin assemblies before NuGetForUnity can label them as analyzers.

Unity requires analyzer DLLs to have plugin platforms disabled and the RoslynAnalyzer asset label set. Without the .dll.meta file in place before AssetDatabase.Refresh, Unity can try to load analyzer assemblies as runtime assemblies and fail on references such as Microsoft.CodeAnalysis.

Changes

• Add shared analyzer .dll.meta generation in AnalyzerHelper
• Write initial analyzer import settings during package install before Unity refreshes assets
• Reuse the same analyzer meta writer from the CLI restore path
• Guard Roslyn-version selection when no analyzer version is supported by the current Unity version
• Update analyzer install test to assert metadata exists before AssetDatabase.Refresh

Verification

• Confirmed locally in Unity that installing MemoryPack/MemoryPack.Generator works without the Microsoft.CodeAnalysis plugin load errors
• dotnet build src\NuGetForUnity.Cli\NuGetForUnity.Cli.sln
• dotnet test src\NuGetForUnity.Cli\NuGetForUnity.Cli.sln --no-build
• dotnet build src\NuGetForUnity.PluginAPI\NuGetForUnity.PluginAPI.sln

[igor84]

Should this change include removal of RoslynAnalyzer code from NugetAssetPostprocessor?

[rafay-pk]

It is an early/import-safe bootstrap, not a replacement for the postprocessor. The new installer-side .meta generation fixes the timing problem: Unity sees correct analyzer import settings before the first refresh/import. But the postprocessor is still useful as the normal Unity-side enforcement path:

• It handles analyzer DLLs that appear without prewritten .meta files, including manual changes or older installs.
• It keeps existing behavior for reimports and asset database processing.
• It adds the NuGetForUnity processed label and applies importer settings through Unity APIs, which is still the most authoritative path once Unity has an importer.
• It avoids making CLI/editor extraction metadata the only source of truth.