feat: Add strong naming assemblies signing

[Blackhex]

Summary

• Adds strong-name signing to all assemblies via Directory.Build.props using Open.snk
• Replaces unsigned internals access with public-key qualified InternalsVisibleTo in *.csproj.
• Removes obsolete AssemblyInfo.cs files, InternalsVisibleTo now handled by *.csproj.

Motivation

• Provides assembly identity stability (publisher verification, GAC compatibility, tamper resistance)
• Unblocks partner/org policies requiring strongly named libraries
• Aligns with NuGet ecosystem expectations for foundational/shared libraries

Linked Issues

• Closes Add strong naming support #147

[Copilot]

Pull Request Overview

This PR implements strong naming for assemblies by enabling assembly signing across the project. It migrates from using AssemblyInfo.cs files to MSBuild project file configurations for managing InternalsVisibleTo attributes with public keys.

• Enables strong naming by configuring SignAssembly and AssemblyOriginatorKeyFile properties
• Migrates InternalsVisibleTo declarations from AssemblyInfo.cs files to MSBuild project files
• Adds public keys to InternalsVisibleTo entries to support signed assemblies

Reviewed Changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
Directory.Build.props	Enables assembly signing for all projects with shared key file configuration
src/A2A/Properties/AssemblyInfo.cs	Removes InternalsVisibleTo attributes (migrated to project file)
src/A2A/A2A.csproj	Adds InternalsVisibleTo entries with public keys for test assemblies
src/A2A.AspNetCore/properties/AssemblyInfo.cs	Removes InternalsVisibleTo attributes (migrated to project file)
src/A2A.AspNetCore/A2A.AspNetCore.csproj	Adds InternalsVisibleTo entry with public key for test assembly

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