Process data write: delimitation of error 702 against 501/502/503 is unclear

[lorandmolnar]

Context

POST /devices/{deviceAlias}/processdata/value declares error 702
(Data set not applicable) and the specific not-applicable codes
501 (I/Q is not configured as DIGITAL_OUTPUT), 502 (C/Q is not
configured as DIGITAL_OUTPUT) and 503 (IO-Link device has no output
process data). Every concrete "this write is not applicable in the
current port configuration" case we could construct falls under one of
the specific codes:

• iqValue write while I/Q is not DO → 501
• cqValue write while C/Q is not DO → 502
• iolink write while the device has no output process data → 503

The remaining candidate — an iolink process data write while the port
is in SIO mode (DIGITAL_INPUT / DIGITAL_OUTPUT), i.e. no IO-Link
communication is running at all — could be read as 702 or as 503.

Tracking on the test specification side: register entry ISS-23
(doc/open_issues.md); analysis in
doc/meetings/WG_7xx_Trigger_Table_2026-08-13.md.

Request

Either state the case error 702 covers on this endpoint (the test
suite will then generate it — our candidate: iolink write on a
SIO-mode port), or remove the 702 declaration in favor of the specific
501/502/503 codes.