Fix non-interactive template uploads

[dheeraj12347]

Summary

Fixes #227.

Template uploads using filepath= failed in non-interactive mode because
UploadFiles() unconditionally used the CLI spinner.

StartSpinner() returns nil when CloudMonkey is running without a shell,
which caused a nil pointer dereference when the spinner was accessed.

This change:

• only creates the spinner when running interactively
• guards spinner operations when no spinner is available
• allows file uploads to continue normally in non-interactive mode
• adds a regression test that performs an actual multipart upload with
  HasShell=false

Validation

• go test ./... ✅
• go vet ./... ✅
• git diff --check ✅
• Regression test for non-interactive upload ✅

Fixes #227

[dheeraj12347]

Hi @jl0815, @DaanHoogland I’ve opened PR #228 for #227. I traced the non-interactive template upload crash to UploadFiles() dereferencing a nil spinner when HasShell=false. The fix makes spinner handling optional while preserving interactive progress behavior, and I added a regression test that performs an actual multipart upload in non-interactive mode. go test ./..., go vet ./..., and git diff --check all pass. Would appreciate your review when you have time.