[build] Drop unused protobuf-net references from Mono.AndroidTools

[jonathanpeppers]

Summary

PR #11529 inlined Mono.AndroidTools from android-platform-support, which carried PackageReferences to protobuf-net 3.2.26 and protobuf-net.Core 3.2.26. The original consumer of these packages was the native tools/base/deploy/installer binary, which was removed in the same PR. None of the inlined managed code (Mono.AndroidTools, Xamarin.AndroidTools, Xamarin.Installer.*, Xamarin.Android.Build.Debugging.Tasks) actually uses the ProtoBuf namespace, [ProtoContract] / [ProtoMember] attributes, or Serializer.* calls — so the references are dead weight.

The actual bug

The unused references were also breaking class-parse's Kotlin metadata parsing on every PR build:

class-parse : warning : Unable to parse Kotlin metadata on 'Utf8("foo/UnsignedInstanceMethods")':
System.IO.FileNotFoundException: Could not load file or assembly
'protobuf-net.Core, Version=3.0.0.0, Culture=neutral, PublicKeyToken=257b51d87d2e4d67'.

…manifesting downstream as:

KotlinUnsignedTypesTests.cs(17,64): error CS1503: Argument 1: cannot convert from 'uint' to 'int'
KotlinUnsignedTypesTests.cs(20,35): error CS0266: Cannot implicitly convert type 'uint' to 'int'.
…

Mono.AndroidTools.csproj outputs to $(MicrosoftAndroidSdkOutDir), where it dropped a v3 protobuf-net.dll (a thin facade) that overwrote the v2 self-contained DLL shipped by Java.Interop's Xamarin.Android.Tools.Bytecode (pinned to protobuf-net 2.4.4). Because protobuf-net.Core.dll was never added to the SDK pack file lists in build-tools/installers/create-installers.targets or Microsoft.Android.Sdk.proj, class-parse failed at runtime and KotlinFixups.Fixup silently skipped Kotlin metadata. Bindings then regenerated unsigned types as their signed counterparts, breaking the hand-written KotlinUnsignedTypesTests.cs.

Fix

Drop the v3 references from src/Mono.AndroidTools/Mono.AndroidTools.csproj (both the PackageReferences and the dependent NuGetFilesToSign group), and the now-unused ProtobufNetVersion / ProtobufNetCoreVersion properties from Directory.Build.props. The v2 protobuf-net.dll from Java.Interop's class-parse becomes the only protobuf-net.dll in $(MicrosoftAndroidSdkOutDir) again, restoring Kotlin metadata parsing.

The existing entries that ship the v2 DLL into the SDK pack — _MSBuildFiles Include="$(MicrosoftAndroidSdkOutDir)protobuf-net.dll" in build-tools/installers/create-installers.targets and <ThirdParty Include="protobuf-net.dll" /> in build-tools/create-packs/SignList.xml, both added by #5529 — continue to do their job.

Verification

• Audited src/Mono.AndroidTools and all transitive consumers: zero usage of using ProtoBuf, [Proto*] attributes, or Serializer.* calls.
• dotnet restore src/Mono.AndroidTools/Mono.AndroidTools.csproj completes cleanly with no transitive protobuf-net resolution.
• KotlinUnsignedTypesTests.cs should compile and pass once class-parse can again deserialize Kotlin metadata.

[Copilot]

Pull request overview

Removes unused protobuf-net v3 package references from the inlined Mono.AndroidTools project to avoid polluting $(MicrosoftAndroidSdkOutDir) with an incompatible protobuf-net.dll, restoring class-parse runtime behavior (Kotlin metadata parsing) and preventing downstream binding/test failures.

Changes:

• Dropped protobuf-net / protobuf-net.Core PackageReferences from src/Mono.AndroidTools/Mono.AndroidTools.csproj, along with the corresponding Release-only NuGetFilesToSign entries.
• Removed the now-dead ProtobufNetVersion / ProtobufNetCoreVersion properties from Directory.Build.props.
• Verified via repository search that no other MSBuild files still reference these removed properties or packages.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Mono.AndroidTools/Mono.AndroidTools.csproj	Removes unused protobuf-net v3 package references and related signing inputs so the project no longer outputs the conflicting facade DLL into the SDK tools output directory.
Directory.Build.props	Deletes unused protobuf-net version properties after removing the only consumer.

[jonathanpeppers]

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