test: expand coverage for runtime resolution, shims, and installers

[CalvinAllen]

Expand test coverage for archive extraction, installers, and integration workflows.

Current State

12 test files exist with coverage in:

• ✅ src/runtimes/python/provider_test.go - Python provider
• ✅ src/runtimes/node/provider_test.go - Node provider
• ✅ src/internal/shim/manager_test.go - Shim management
• ✅ src/internal/runtime/version_test.go, registry_test.go - Runtime infrastructure
• ✅ src/cmd/migrate_test.go, uninstall_test.go - Command tests
• ✅ src/internal/config/paths_test.go, version_test.go - Config tests

Remaining Work

Missing Unit Tests

• src/internal/download/extract_test.go - Test ExtractZip, ExtractTarGz, ZipSlip protection
• src/internal/download/download_test.go - Test HTTP download logic
• src/cmd/shim/ - Shim path resolution edge cases

Missing Integration Tests

• Installer dry-run mode (install.sh --dry-run, install.ps1 -DryRun)
• Full install → configure → execute workflow
• Migration workflow (detect → migrate → verify)
• Reshim workflow (install package → reshim → verify new shim)

CI Integration

• Add testable mode to installers for CI validation
• Platform-specific test matrix (Windows, macOS, Linux)

Acceptance Criteria

• Archive extraction has unit tests (including security edge cases)
• Installers have a testable mode exercised in CI
• Shim resolution covered by platform-specific tests