Fix isElementStreamedIn returning false for localPlayer (multitheftauto#4836)

#### Summary

Fixes isElementStreamedIn(localPlayer) returning false on the client in
some contexts. The local player is, by definition, always streamed in
from its own client's perspective returning false here is a contract
break.

Fix is scoped to the Lua binding (CLuaElementDefs::IsElementStreamedIn):
when the queried element is the local player, return true directly.
Internal C++ uses of IsStreamedIn() are untouched, so no engine-side
behavior change.


```
if (IS_PLAYER(pEntity) && static_cast<CClientPlayer*>(pEntity)->IsLocalPlayer())
{
    lua_pushboolean(luaVM, true);
    return 1;
}
```

#### Motivation

This worked in 1.6 and broke in 1.7. The local player is created through
a dedicated path (_CreateLocalModel) that doesn't go through the regular
streamer's stream-in flow, so its internal m_bStreamedIn flag stays
false by default. In 1.6 the old streamer happened to flip it on as a
side effect of how it sorted/picked elements every frame; after the
streamer refactor that side effect is gone.

The bug doesn't always reproduce, in normal gameplay the streamer keeps
pulling the local player back in often enough that the flag ends up true
and nothing looks broken. But in less typical setups (custom
interior/dimension, camera framed away from the ped, idle ped, basically
any in-game UI that takes over the camera and parks the player aside,
like a garage), the streamer doesn't reach the local player and the flag
stays false permanently.

The visible impact is that this very common pattern silently stops
working in those contexts:

```
setElementData(localPlayer, "mykey", data)

addEventHandler("onClientElementDataChange", root, function(dataName)
    if dataName == "mykey" and isElementStreamedIn(source) then
        -- update something
    end
end)
```

The event still fires, but the isElementStreamedIn(source) guard now
drops every update because source is localPlayer. We hit it on multiple
unrelated systems in our resource (garage body parts preview, window
tints preview), everything works in the open world, breaks the moment
the player enters our garage.

#### Test plan

Reproducer:

Reproducing the actual bug requires a specific in-game UI context
(custom dimension/interior + camera moved off the player), which isn't
easy to share as a minimal snippet. The behavioral check itself is
simple in any context where you can call it from the

local client:

isElementStreamedIn(localPlayer)

Before the fix: can return false depending on context.

After the fix: always returns true for the local player.

#### Checklist

* [X] Your code should follow the [coding
guidelines](https://wiki.multitheftauto.com/index.php?title=Coding_guidelines).
* [X] Smaller pull requests are easier to review. If your pull request
is beefy, your pull request should be reviewable commit-by-commit.

Author: naouh

Client/mods/deathmatch/logic/luadefs/CLuaElementDefs.cpp

@@ -1592,6 +1592,16 @@ int CLuaElementDefs::IsElementStreamedIn(lua_State* luaVM)
         // Is this a streaming compatible class?
         if (pEntity->IsStreamingCompatibleClass())
         {
+            // Local player is always streamed in from its own perspective.
+            // Its entity is created via _CreateLocalModel() which doesn't go through
+            // InternalStreamIn(), so m_bStreamedIn can stay false even though the player
+            // is fully active — breaking scripts that filter handlers with isElementStreamedIn(source).
+            if (IS_PLAYER(pEntity) && static_cast<CClientPlayer*>(pEntity)->IsLocalPlayer())
+            {
+                lua_pushboolean(luaVM, true);
+                return 1;
+            }
+
             CClientStreamElement* pStreamElement = static_cast<CClientStreamElement*>(pEntity);
 
             // Return whether or not this class is streamed in