[r] Use libtiledbsoma for the mode accessor

[johnkerl]

Currently this uses R6 class-private state. It should use libtiledbsoma's mode accessor.

Note that this requires as a prerequisite #3061 since only after we have a statefully open array handle can we possibly query it for mode.

As narrated on #3061) , as of 2024-09-23, to get mode we have to open the libtiledbsoma handle, ask for its mode, and close the handle -- and to open that handle we have to know the mode we want to open with -- which comes from class-private state -- which is circular.

Therefore #3061 blocks this.

#2406

[sc-55688]