* Support Tart Guest Agent transport for VM startup scripts
* TestTartGuestAgentStartupScript: make ScriptContent more readable
* Align Tart Guest Agent's startup script execution with SSH
* Use mvdan.cc/sh/v3/syntax to validate "export" shell commands
* Use Bash instead of Zsh on Linux
* Invert logic when validating vm.StartupScript.Transport for simplicity
* Document startup_script's transport field in OpenAPI specification
* Prevent unbounded buffering of startup script output
* grpc.WaitForReady(true) → retry.DoWithData()
