Avoid crashing on unexpected localized key storage

[ZHUOLIN0928]

Summary

• Replace force casts in LocalizedStringKey reflection with guarded casts and safe argument filtering.
• Add a regression test covering an interpolated AttributedString storage.

Fixes #269

Testing

• git diff --check
• swiftc -parse passed for the changed source and test files
• swift build --target SwiftNavigation was blocked by the existing SwiftUIMacros.StateMacro plugin failure in the current CommandLineTools environment