Add client-local routing for keybind and plugin pipes

[yamam]

Summary

This PR adds optional client-local routing for keybind and
plugin-to-plugin pipes.
Closes #5392.

Changes

• Add a client_local option to MessagePlugin keybinds.

  bind "Alt l" {
      MessagePlugin "vim-tab-conditional" {
          name "right"
          client_local true
      }
  }

• Add client_local to MessageToPlugin.

  MessageToPlugin::new("message-name")
      .with_plugin_url("destination-plugin")
      .client_local()

• When enabled, deliver the pipe only to the matching plugin instance
  associated with the originating client.
• Preserve the existing broadcast behavior when client_local is omitted or
  set to false.
• Exclude plugin instances associated with disconnected clients from CLI pipe
  delivery.
• Treat client_local as a routing option rather than part of the destination
  plugin configuration.

Compatibility

client_local defaults to false, so existing keybindings and plugins retain
their current broadcast behavior.
CLI pipes do not support client_local because they are sent through a
short-lived CLI client rather than a particular attached client.