InputText: fixed a bug (regression in 1.91.2) where modifying text buffer within a callback would sometimes prevents further appending to the buffer. (#7925)

There's a confusion between TextA.Size and CurLenA we should be merging them.
Amend 19accb14a

imgui_internal.h

@@ -1111,7 +1111,7 @@ struct IMGUI_API ImGuiInputTextState
     ImStbTexteditState*     Stb;                    // State for stb_textedit.h
     ImGuiID                 ID;                     // widget id owning the text state
     int                     CurLenA;                // UTF-8 length of the string in TextA (in bytes)
-    ImVector<char>          TextA;                  // main UTF8 buffer.
+    ImVector<char>          TextA;                  // main UTF8 buffer. TextA.Size is a buffer size! Should always be >= buf_size passed by user (and of course >= CurLenA + 1).
     ImVector<char>          InitialTextA;           // value to revert to when pressing Escape = backup of end-user buffer at the time of focus (in UTF-8, unaltered)
     ImVector<char>          CallbackTextBackup;     // temporary storage for callback to support automatic reconcile of undo-stack
     int                     BufCapacityA;           // end-user buffer capacity