NextAction refactoring to eliminate sentinel arrays and pointers (#1923)

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# Description

This PR aims to refactor the NextAction declaration to achieve two
goals:

## Eliminate C-style sentinel arrays

Currently, a double pointer (`NextAction**`) approach is being used.
This an old pre-C++11 (< 2011) trick before `std::vector<>` became a
thing.
This approach is painful for developers because they constantly need to
declare their `NextAction` arrays as:
```cpp
NextAction::array(0, new NextAction("foo", 1.0f), nullptr)
```
Instead of:
```cpp
{ new NextAction("foo", 1.0f) }
```
The first argument of `NextAction::array` is actually a hack. It is used
to have a named argument so `va_args` can find the remaining arguments.
It is set to 0 everywhere but in fact does nothing. This is very
confusing to people unfamiliar with this antiquated syntax.
The last argument `nullptr` is what we call a sentinel. It's a `nullptr`
because `va_args` is looking for a `nullptr` to stop iterating. It's
also a hack and also leads to confusion.

## Eliminate unnecessary pointers for `NextAction`

Pointers can be used for several reasons, to cite a few:
- Indicate strong, absolute identity.
- Provide strong but transferable ownership (unlike references).
- When a null value is acceptable (`nullptr`).
- When copy is expensive.

`NextAction` meets none of these criteria:
- It has no identity because it is purely behavioural.
- It is never owned by anything as it is passed around and never fetched
from a registry.
- The only situations where it can be `nullptr` are errors that should
in fact throw an `std::invalid_argument` instead.
- They are extremely small objects that embark a single `std::string`
and a single `float`.

Pointers should be avoided when not strictly necessary because they can
quickly lead to undefined behaviour due to unhandled `nullptr`
situations. They also make the syntax heavier due to the necessity to
constantly check for `nullptr`. Finally, they aren't even good for
performance in that situation because shifting a pointer so many times
is likely more expensive than copying such a trivial object.

# End goal

The end goal is to declare `NextAction` arrays this way:
```cpp
{ NextAction("foo", 1.0f) }
```

> [!NOTE]
> Additional note: `NextAction` is nothing but a hacky proxy to an
`Action` constructor. This should eventually be reworked to use handles
instead of strings. This would make copying `NextAction` even cheaper
and remove the need for the extremely heavy stringly typed current
approach. Stringly typed entities are a known anti-pattern so we need to
move on from those.

src/strategy/Trigger.h

@@ -3,8 +3,7 @@
  * and/or modify it under version 3 of the License, or (at your option), any later version.
  */
 
-#ifndef _PLAYERBOT_TRIGGER_H
-#define _PLAYERBOT_TRIGGER_H
+#pragma once
 
 #include "Action.h"
 #include "Common.h"
@@ -15,7 +14,11 @@ class Unit;
 class Trigger : public AiNamedObject
 {
 public:
-    Trigger(PlayerbotAI* botAI, std::string const name = "trigger", int32 checkInterval = 1);
+    Trigger(
+        PlayerbotAI* botAI,
+        const std::string name = "trigger",
+        int32_t checkInterval = 1
+    );
 
     virtual ~Trigger() {}
 
@@ -23,7 +26,7 @@ public:
     virtual void ExternalEvent([[maybe_unused]] std::string const param, [[maybe_unused]] Player* owner = nullptr) {}
     virtual void ExternalEvent([[maybe_unused]] WorldPacket& packet, [[maybe_unused]] Player* owner = nullptr) {}
     virtual bool IsActive() { return false; }
-    virtual NextAction** getHandlers() { return nullptr; }
+    virtual std::vector<NextAction> getHandlers() { return {}; }
     void Update() {}
     virtual void Reset() {}
     virtual Unit* GetTarget();
@@ -33,32 +36,49 @@ public:
     bool needCheck(uint32 now);
 
 protected:
-    int32 checkInterval;
-    uint32 lastCheckTime;
+    int32_t checkInterval;
+    uint32_t lastCheckTime;
 };
 
 class TriggerNode
 {
 public:
-    TriggerNode(std::string const name, NextAction** handlers = nullptr)
-        : trigger(nullptr), handlers(handlers), name(name)
-    {
-    }  // reorder args - whipowill
-
-    virtual ~TriggerNode() { NextAction::destroy(handlers); }
+    TriggerNode(
+        const std::string& name,
+        std::vector<NextAction> handlers = {}
+    ) :
+    trigger(nullptr),
+    handlers(std::move(handlers)),
+    name(name)
+    {}
 
     Trigger* getTrigger() { return trigger; }
     void setTrigger(Trigger* trigger) { this->trigger = trigger; }
-    std::string const getName() { return name; }
+    const std::string getName() { return name; }
 
-    NextAction** getHandlers() { return NextAction::merge(NextAction::clone(handlers), trigger->getHandlers()); }
+    std::vector<NextAction> getHandlers()
+    {
+        std::vector<NextAction> result = this->handlers;
 
-    float getFirstRelevance() { return handlers[0] ? handlers[0]->getRelevance() : -1; }
+        if (trigger != nullptr)
+        {
+            std::vector<NextAction> extra = trigger->getHandlers();
+            result.insert(result.end(), extra.begin(), extra.end());
+        }
+
+        return result;
+    }
+
+    float getFirstRelevance()
+    {
+        if (this->handlers.size() > 0)
+            return this->handlers[0].getRelevance();
+
+        return -1;
+    }
 
 private:
     Trigger* trigger;
-    NextAction** handlers;
-    std::string const name;
+    std::vector<NextAction> handlers;
+    const std::string name;
 };
-
-#endif