refactor: migrate common/client/im to FileIO and add localfileio tests (#322)

* 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

cmd/service/service.go

@@ -250,6 +250,7 @@ func serviceMethodRun(opts *ServiceMethodOptions) error {
 		JqExpr:     opts.JqExpr,
 		Out:        out,
 		ErrOut:     f.IOStreams.ErrOut,
+		FileIO:     f.ResolveFileIO(opts.Ctx),
 		CheckError: checkErr,
 	})
 }