Fix duplicate signal registration with mining mob respawners (#96827)

## About The Pull Request

So it's possible for a mining mob respawner to be spawned by
`/datum/biome/populate_turfs`, which calls `setup()` on the respawner,
and if the mob being passed is a random spawner, it'll register
`COMSIG_ATOM_AFTER_SUCCESSFUL_INITIALIZED_ON` on the turf to wait for
that random spawner to spawn something. Since this is happening prior to
atom subsystem init, that spawn won't happen until later. Once the atom
subsystem starts Initializing stuff, if the mining respawner gets
Initialized before the random spawner, it'll call `spawn_mob` which, if
it happens to choose the same turf its on, will register
`COMSIG_ATOM_AFTER_SUCCESSFUL_INITIALIZED_ON` again and runtime.

## Why It's Good For The Game

<img width="1388" height="310" alt="image"
src="https://github.com/user-attachments/assets/90bc2030-b8e4-4e1a-92cd-bbd399359684"
/>

## Changelog

N/A

code/game/objects/effects/mining_mob_respawner.dm

@@ -5,6 +5,8 @@
 	invisibility = INVISIBILITY_ABSTRACT
 	/// Do we check the outdoorsness of our spawn tile?
 	var/outdoor_only = TRUE
+	/// Are we waiting for a mob to spawn so we can link to it?
+	var/registered_spawn_signal = FALSE
 	// Spawn somewhere in an area around the spawner rather than dead on it
 	var/respawn_range = 3
 	/// Min time from storm to spawn a mob
@@ -38,7 +40,7 @@
 				filtered_mobs += path
 		valid_mobs = filtered_mobs
 
-	if (!our_mob?.resolve())
+	if (!our_mob?.resolve() && !registered_spawn_signal)
 		make_mob()
 
 	// We're just going to go ahead and assume these won't move after being spawned
@@ -51,6 +53,7 @@
 
 	if (istype(initial_spawn, /obj/effect/spawner/random))
 		RegisterSignal(get_turf(src), COMSIG_ATOM_AFTER_SUCCESSFUL_INITIALIZED_ON, PROC_REF(get_spawned_mob))
+		registered_spawn_signal = TRUE
 	else
 		register_spawn(src, initial_spawn)
 
@@ -85,6 +88,7 @@
 	var/turf/spawn_turf = pick(valid_locations)
 	// Bit roundabout but it's the only way of intercepting mob spawners
 	RegisterSignal(spawn_turf, COMSIG_ATOM_AFTER_SUCCESSFUL_INITIALIZED_ON, PROC_REF(get_spawned_mob))
+	registered_spawn_signal = TRUE
 	new spawn_path(spawn_turf)
 
 /// Intercept the next mob spawned on the turf, because we might have spawned an object which spawns a mob instead
@@ -94,6 +98,7 @@
 		return
 
 	UnregisterSignal(source, COMSIG_ATOM_AFTER_SUCCESSFUL_INITIALIZED_ON)
+	registered_spawn_signal = FALSE
 	register_spawn(source, new_spawn)
 	play_spawn_animation(new_spawn, source)