Invalid bracket encoding reported when using square brackets around strings

Libadalang appears to treat any single string wrapped in square brackets as invalid, reporting "invalid bracket encoding". See the following valid code as an example:

pragma Ada_2022;

with Ada.Containers.Vectors;

procedure Example is
   type String_1 is new String (1 .. 1);

   package String_Vec is new Ada.Containers.Vectors (Positive, String_1);
   package Integer_Vec is new Ada.Containers.Vectors (Positive, Integer);

   V1 : String_Vec.Vector := ["a", "b"];
   V2 : Integer_Vec.Vector := [1];
   V3 : String_Vec.Vector := ["a"]; --  Error here
begin
   null;
end Example;

This has only been tested against the version of libadalang included in ALS 2026.0.202510141.

Start by reproducing the provided Ada 2022 example with the libadalang version included in ALS 2026.0.202510141, focusing on the V3 declaration and its reported error. The fix is complete when the valid single-string square-bracket aggregate is accepted without an "invalid bracket encoding" diagnostic.