fix: avoid double-zipping build artifacts

[Soner (shyim)]

Summary

• upload the generated extension ZIP directly with upload-artifact v7
• avoid wrapping the extension ZIP in another artifact archive
• document the resulting single-ZIP behavior

Validation

• build-zip action definition check
• git diff --check
• bash downstream/wait.test.bash
• bash setup-extension/action.test.bash
• bash version-bump/version-bump.test.bash