BUG: Interchange protocol uses `u` for string format code but offets are 8 bytes

[WillAyd]

Pandas version checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

When inspecting the offsets buffer of a string sent via the interchange protocol the buffer uses 8 bytes per entry to store an offset even when the format code is "u"

LargeString should use 8 bytes per entry, but has a different format code of "U"

Issue Description

see above

Expected Behavior

u should have 4 byte offsets, U should have 8

Installed Versions

main

[MarcoGorelli]

closed by #56772