Add opt-in running-session picker for coder_app terminals #25058

Clicking a coder_app terminal button creates a new PTY each time instead of reconnecting to the same session, which causes terminal and TUI state to be lost.

Add an opt-in behavior for coder_app terminals that shows the user's running sessions and lets them choose one to reconnect to, rather than always starting a new PTY. This preserves terminal and TUI state when an existing session is selected. When there are no running sessions, launching the app creates a new PTY, matching the current behavior.

resource "coder_app" "claude" {
   agent_id                  = coder_agent.dev.id
   slug                      = "claude"
   display_name              = "Claude"
   command                   = "claude"
   show_running_sessions     = true // optional; defaults to false
 }