Add checks for test.sh existence and executable permissions in build command.

[rohitpaulk]

Note

Adds early checks ensuring testerDir/test.sh exists and is executable, with clear failure messages.

• Build command (builder/internal/commands/build_image_command.go):
  • Validate testerDir/test.sh exists; return error if missing.
  • Check test.sh is executable; return error if not.
  • Handle and surface errors from stat on test.sh with detailed messages.

^{Written by Cursor Bugbot for commit 8bc68f6. This will update automatically on new commits. Configure here.}

[cursor]

Bug: Missing check that test.sh is a regular file

The validation checks existence and executable permissions but doesn't verify that test.sh is a regular file. If test.sh is a directory (which typically has execute bits set for traversal), the check fileInfo.Mode()&0111 == 0 passes, but attempting to execute it as a script later will fail. Adding a check like fileInfo.Mode().IsRegular() would catch this case and provide a clearer error message upfront.