test(arch): Add unit tests for arch package - #2703

Prerequisite checklist

• Read the contribution guidelines regarding submitting new changes to the project;
• Tested your changes locally.

Description of changes

Summary

• Adds comprehensive unit tests for unikraft/arch package
• Achieves 97% coverage (100% on all files except platform-specific host.go)

Functions tested

• ArchitectureName.String, ArchitectureByName, ArchitecturesByName
• Architectures, ArchitectureAliases
• NewArchitectureFromSchema — empty string error + valid name
• All ArchitectureConfig accessors: Name, String, Source, Version, Type, Path
• IsUnpacked — existing dir, non-existent path, empty path
• KConfig — correct CONFIG_ARCH_* key for x86_64, arm64, arm, unknown
• MarshalYAML, KConfigTree, PrintInfo
• TransformFromSchema — string, invalid type, with UK_BASE context
• HostArchitecture — current host arch
• All options: WithName, WithVersion, WithSource, WithPath, WithKConfig

Note

host.go reaches 60% coverage — the arm/default branches of
HostArchitecture are platform-specific and cannot be tested on amd64.

Related Issue

Closes #2702

Test plan

• Run go test kraftkit.sh/unikraft/arch -cover — 97% coverage, all PASS