at_qa_mode is missing

Issue in ./help/interface/data-collection/cookies/web-sdk.md

Since WebSDK can respond with Target's qa cookie, we should mention the at_qa_mode as well.

Mentioned here:
https://experienceleague.adobe.com/en/docs/platform-learn/migrate-target-to-websdk/debugging
https://experienceleague.adobe.com/en/docs/target/using/activities/activity-qa/activity-qa
https://wiki.corp.adobe.com/display/dipaemeateam/Target+-+Cookies

Open help/interface/data-collection/cookies/web-sdk.md and review the existing Target cookie descriptions. Read the linked Adobe Target QA references for the expected at_qa_mode wording, then document that cookie alongside the others. Done means the WebSDK cookie documentation mentions at_qa_mode accurately and consistently.

Written by the indexing model from the issue text.