The authoritative source here is the United Nations System of National Accounts, SNA.
Change in inventories are indeed part of what we normally call Investment. First, for clarification, the definition of "change in inventories", taken from page 108 of SNA, version 2008:
The basic principle underlying the measurement of changes
in inventories of finished goods is that output should be
recorded at the time it is produced and valued at the same
price whether it is sold, otherwise used or entered into
inventories for sale or use later. In effect, goods only enter
inventories when they are not immediately used
for sale or other use in the period they
are produced. Similarly, goods are withdrawn from inventories when the demand for the
goods exceeds the amount produced in a period. No output
is recorded when goods produced previously are withdrawn
from inventories and sold or otherwise used unless a
storage activity as described be low in section F takes place.
Second, the components of "Investment" (officially called Gross capital formation), as stated in page 282 of the aforementioned document is:
There are three types of capital formation to be examined,
gross fixed capital formation,
changes in inventories and
acquisition less disposal of valuables.
This and following pages go about explaining each of them, and how to measure them.
Finally, the document provides a worked example of the table of "uses of output" (the expenditure approach of GDP). Here is a screenshot of a section of that worked example, presented in page 291. You can see that change in inventories are indeed treated as part of "Gross Capital Formation":