Avoid breaking change on Libadalang.Auto_Provider.Get_Unit and Libadalang.Project_Provider.Get_Unit

Integrating libadalang 25.0.0 from libadalang 23.0.0

Calls to Libadalang.Auto_Provider.Get_Unit and Libadalang.Project_Provider.Get_Unit explicitly raise Program_Error exception.
Previously, developer was able to get a unit through the call

declare
  Unit : constant Libadalang.Analysis.Analysis_Unit'Class := LAL_Auto_Unit_Provider.Get_Unit
           (Context        => LAL_Context,
            Name           => Name,
            Kind           => Kind);

within the context below

declare
   LAL_Auto_Unit_Provider : Libadalang.Auto_Provider.Auto_Unit_Provider;
   LAL_Context            : Libadalang.Analysis.Analysis_Context;
begin
   LAL_Auto_Unit_Provider := Libadalang.Auto_Provider.Create_Auto_Provider (Input_Files => Files);
   Unit_Provider_Ref      := LAL_Auto_Unit_Provider.Create_Unit_Provider_Reference;
   LAL_Context            := Libadalang.Analysis.Create_Context (Unit_Provider => Unit_Provider_Ref);

This breaking change is related to the implementation of "PLE root" Langkit mechanism (Commit 1ab5c03 [1])

In order to maintain compatibility, Libadalang.Auto_Provider.Get_Unit and Libadalang.Project_Provider.Get_Unit could simply call the class underlying Get_Unit_And_PLE_Root, with the additional parameters Unit and PLE_Root_Index declared in the Get_Unit function scope, as proposed below instead of explicitly raising a Program_Error exception

 overriding function Get_Unit
   (Provider    : Auto_Unit_Provider;
    Context     : Analysis_Context'Class;
    Name        : Text_Type;
    Kind        : Analysis_Unit_Kind;
    Charset     : String := "";
    Reparse     : Boolean := False) return Analysis_Unit'Class
 is
    Unit           : Analysis_Unit := No_Analysis_Unit;
    PLE_Root_Index : Natural       := 0;
 begin
    Provider.Get_Unit_And_PLE_Root
      (Context        => Context,
       Name           => Name,
       Kind           => Kind,
       Charset        => Charset,
       Reparse        => Reparse,
       Unit           => Unit,
       PLE_Root_Index => PLE_Root_Index);
    return Unit;
 end Get_Unit;

[1]
https://github.com/AdaCore/libadalang/commit/1ab5c037a8ac3336e3c822a78484e79e1287dbb7#diff-405ad85e9a2303876561467d05a053fd2d5173d93fc6abd0bd55fd84d36d57c1

Start with Libadalang.Auto_Provider.Get_Unit and Libadalang.Project_Provider.Get_Unit, then inspect the underlying Get_Unit_And_PLE_Root implementation introduced by commit 1ab5c03. Confirm how the Unit and PLE_Root_Index outputs are handled. Done means both public Get_Unit calls preserve the previous behavior instead of explicitly raising Program_Error.