* refactor: migrate common/client/im to FileIO and add localfileio tests
- runner resolveInputFlags: replace validate.SafeInputPath + vfs.ReadFile
with FileIO.Open + io.ReadAll
- SaveResponse: delegate to FileIO.Save + ResolvePath
- cmd/api, cmd/service: pass FileIO to ResponseOptions
- im: replace validate.SafeLocalFlagPath with RuntimeContext.ValidatePath,
migrate download/upload to FileIO.Save/Open/Stat
- Add path_test.go and atomicwrite_test.go for localfileio
- Add validate_media_test.go for im media flag validation
- Adapt test mocks to fileio.FileInfo interface
* feat: add FileIO extension for file transfer abstraction
Introduce extension/fileio package with Provider/FileIO/File interfaces
and a global registry, following the same pattern as extension/credential.
- Add LocalFileIO default implementation with path validation and atomic writes
- Wire FileIOProvider into Factory and resolve at runtime via RuntimeContext.FileIO()
- Factory holds Provider (not resolved instance), deferring resolution to execution time
SunPeiYang996