app-server: make thread/shellCommand tests shell-aware

[bolinfest]

Why

thread/shellCommand executes the raw command string through the current user shell, which is PowerShell on Windows. The two v2 app-server tests in app-server/tests/suite/v2/thread_shell_command.rs used POSIX printf, so Bazel CI on Windows failed with printf not being recognized as a PowerShell command.

For reference, the user-shell task wraps commands with the active shell before execution: core/src/tasks/user_shell.rs.

What Changed

Added a test-local helper that builds a shell-appropriate output command and expected newline sequence from default_user_shell():

• PowerShell: Write-Output '...' with \r\n
• Cmd: echo ... with \r\n
• POSIX shells: printf '%s\n' ... with \n

Both thread_shell_command_runs_as_standalone_turn_and_persists_history and thread_shell_command_uses_existing_active_turn now use that helper.

Verification

• cargo test -p codex-app-server thread_shell_command