#5599 `#{session_activity_flag}` is `0` when activity is present only in a non-current window.

Issue description

#{session_activity_flag} is 0 when activity is present only in a non-current window. The manual describes this format as:

1 if any window in session has activity

This affects uses such as list-sessions -F, status formats, and the activity marker in choose-tree.

Below is the process for reproducing this issue. In the example below, the noisy window has window_activity_flag=1, but the session containing it has session_activity_flag=0 because quiet is the current window. Reproduction

tmux -Ltest kill-server 2>/dev/null

tmux -Ltest -f/dev/null new-session -d -s act -x 80 -y 24 \
    -n quiet 'exec sleep 3600'
tmux -Ltest -f/dev/null new-window -d -t act: \
    -n noisy 'exec sleep 3600'
tmux -Ltest -f/dev/null select-window -t act:quiet
tmux -Ltest -f/dev/null set-window-option -t act:quiet monitor-activity off
tmux -Ltest -f/dev/null set-window-option -t act:noisy monitor-activity on

tmux -Ltest -f/dev/null respawn-pane -k -t act:noisy \
    'sh -c "echo ACTIVITY-POC; exec sleep 3600"'
sleep 0.3

tmux -Ltest -f/dev/null display-message -p -t act:quiet '#{window_activity_flag}'
tmux -Ltest -f/dev/null display-message -p -t act:noisy '#{window_activity_flag}'
tmux -Ltest -f/dev/null display-message -p -t act: '#{session_activity_flag}'
tmux -Ltest -f/dev/null list-sessions \
    -F '#{session_name} #{session_activity_flag}'

tmux -Ltest kill-server

Expected output:

0
1
1
act 1

Actual output:

0
1
0
act 0

Required information

tmux version (tmux -V): tmux next-3.9
Platform (uname -sp): Linux x86_64
Terminal in use (xterm, rxvt, etc): VS Code integrated terminal (xterm.js)
$TERM inside tmux (echo $TERM): xterm-256color
$TERM outside tmux (echo $TERM): xterm-256color
Logs from tmux (tmux kill-server; tmux -vv new).

1789382546.786501 cmd_unpack_argv: argv[0]=new-session
1789382546.786802 cmd_unpack_argv: argv[1]=-d
1789382546.786863 cmd_unpack_argv: argv[2]=-s
1789382546.786887 cmd_unpack_argv: argv[3]=act
1789382546.786910 cmd_unpack_argv: argv[4]=-x
1789382546.786933 cmd_unpack_argv: argv[5]=80
1789382546.786956 cmd_unpack_argv: argv[6]=-y
1789382546.786978 cmd_unpack_argv: argv[7]=24
1789382546.787002 cmd_unpack_argv: argv[8]=-n
1789382546.787024 cmd_unpack_argv: argv[9]=quiet
1789382546.787047 cmd_unpack_argv: argv[10]=exec sleep 3600
1789382546.787790 cmd_parse_build_commands: new-session -d -n quiet -s act -x 80 -y 24 "exec sleep 3600"
1789382546.788035 cmdq_fire_command </dev/pts/4>: (3577) new-session -d -n quiet -s act -x 80 -y 24 "exec sleep 3600"
1789382546.791528 new session act $1
1789382546.792241 spawn_window: name=quiet, flags=0
1789382546.794094 window_create: @1 create 80x24 (16x32)
1789382546.806390 events_fire: pane-created: created_empty=0, created_respawn=0, event=pane-created, pane=%1, pane_command="exec sleep 3600", pane_current_path=/home/asfdf/Desktop/issues/tumx/test, session=$1, window=@1, window_index=0
1789382546.820605 events_fire: window-created: event=window-created, window=@1
1789382546.828659 events_fire: session-created: event=session-created, session=$1
1789382546.831025 events_fire: after-new-session: _cmdq_item=0x55820b901600, argument_0=exec sleep 3600, arguments=-d -n quiet -s act -x 80 -y 24 "exec sleep 3600", event=after-new-session
1789382546.837496 lost client 0x55820b944ab0

1789382546.873148 cmd_unpack_argv: argv[0]=new-window
1789382546.873195 cmd_unpack_argv: argv[1]=-d
1789382546.873219 cmd_unpack_argv: argv[2]=-t
1789382546.873242 cmd_unpack_argv: argv[3]=act:
1789382546.873266 cmd_unpack_argv: argv[4]=-n
1789382546.873289 cmd_unpack_argv: argv[5]=noisy
1789382546.873315 cmd_unpack_argv: argv[6]=exec sleep 3600
1789382546.874148 cmd_parse_build_commands: new-window -d -n noisy -t act: "exec sleep 3600"
1789382546.874404 cmdq_fire_command </dev/pts/4>: (3581) new-window -d -n noisy -t act: "exec sleep 3600"
1789382546.884298 events_fire: pane-created: created_empty=0, created_respawn=0, event=pane-created, pane=%2, pane_command="exec sleep 3600", pane_current_path=/home/asfdf/Desktop/issues/tumx/test, session=$1, window=@2, window_index=1
1789382546.892134 events_fire: window-created: event=window-created, window=@2
1789382546.893648 events_fire: window-linked: event=window-linked, session=$1, window=@2, window_index=1
1789382546.895253 events_fire: after-new-window: _cmdq_item=0x55820b901600, argument_0=exec sleep 3600, arguments=-d -n noisy -t act: "exec sleep 3600", event=after-new-window
1789382546.898413 lost client 0x55820b944ab0

1789382546.938426 cmd_unpack_argv: argv[0]=select-window
1789382546.938449 cmd_unpack_argv: argv[1]=-t
1789382546.938473 cmd_unpack_argv: argv[2]=act:quiet
1789382546.938678 cmd_parse_build_commands: select-window -t act:quiet
1789382546.939045 cmdq_fire_command </dev/pts/4>: (3585) select-window -t act:quiet
1789382546.939771 events_fire: after-select-window: _cmdq_item=0x55820b90bf80, arguments=-t act:quiet, event=after-select-window, flag_t=act:quiet, flag_t_0=act:quiet
1789382546.944163 lost client 0x55820b944ab0