fix: drop stale tmux commands after SSH exit

[oz-for-oss]

Closes #12215

Summary

• Drop tmux control-mode commands in PtyController when tmux control mode is no longer active.
• Prevent stale background tmux commands, including new-window, from being written into the local shell after an SSH session exits.
• Add focused unit tests for inactive-control-mode dropping and active-control-mode formatting.

Validation

• ./script/format
• CARGO_BUILD_JOBS=1 cargo test -p warp drops_tmux_command_when_control_mode_is_inactive --lib
• CARGO_BUILD_JOBS=1 cargo test -p warp formats_tmux_command_when_control_mode_is_active --lib
• CARGO_BUILD_JOBS=1 cargo clippy -p warp --lib -- -D warnings

Notes

• A broader cargo nextest run -p warp -E 'test(pty_controller)' attempt was killed by SIGKILL during compilation in the sandbox, so the new tests were rerun individually with one cargo build job.
• cargo clippy -p warp --all-targets --all-features --tests -- -D warnings was blocked by a missing generated preview_config.json for the preview binary test target; library clippy passed.