test from MediawikiEditEntityTest that result in a non-OK status:

• testEditConflict -- returns non-OK status
• testAttemptSaveWithLateConflict -- returns non-OK status
• testCheckEditPermissions -- covered via MediaWiki REST framework
• testAttemptSavePermissions -- covered via MediaWiki REST framework
• testCheckLocalEntityTypes -- always local
• testAttemptSaveRateLimit -- covered via ItemUpdatePrevented
• testIsTokenOk -- $token is always false (i.e. not used)
• testAttemptSaveUnresolvedRedirect -- covered via metadata redirect check

non-OK status updates in MediawikiEditEntity:

• L677 -- not applicable, since Item is always local
• L684 -- not applicable, due to if ( $token !== false ... )
• L688 -- not applicable, since status is still good
• L692 -- not applicable, since MediaWiki REST framework covers edit permission checks
• L694 -- rate limits will result in non-OK status
• L714 -- not applicable, since redirects are covered by our metadata check
• L734 -- edit conflicts will result in non-OK status
• L749, L785 -- EntityContentTooBigException will end up in our UnexpectedErrorHandler and cause a 'wikibase-error-entity-too-big' error message in the exception channel

Whether any additional third-party extensions can cause a non-OK status via editFilterHook, I did not cover in this investigation. For now it should be safe to assume those are indeed "unexpected errors" which will end up in our generic error handler.

As a result, in addition to the spam-blocklist, abusefilter and rate limits we need to cover failed save attempts due to:

• edit conflicts,
• edits that will exceed the item size limit.

LGTM, thanks for investigating and for the good notes @Silvan_WMDE! \o/