Stabilize zsh fork app-server tests

[aibrahim-oai]

What changed

• turn_start_shell_zsh_fork_executes_command_v2 now keeps the shell command alive with a file marker until the interrupt arrives instead of using a command that can finish too quickly.
• turn_start_shell_zsh_fork_subcommand_decline_marks_parent_declined_v2 now waits for turn/completed before sending a fallback interrupt and accepts the real terminal outcomes observed across platforms.

Why this fixes the flake

• The original tests assumed a narrow ordering window: the child command would still be running when the interrupt happened, and completion would always arrive in one specific order.
• In CI, especially across different shells and runner speeds, those assumptions break. Sometimes the child finishes before the interrupt; sometimes the protocol completes while the fallback path is still arming itself.
• Holding the command open until the interrupt and waiting for the explicit protocol completion event makes the tests synchronize on the behavior under test instead of on wall-clock timing.

Scope

• Test-only change.