POST behaviour unchanged (JSON body). GET on the same URL now triggers the
workflow with the query string collapsed into a JSON object: /wh/wf/hi?a=1&name=x
becomes {"a":"1","name":"x"}, and repeated keys collapse into a string
array (?tag=a&tag=b → {"tag":["a","b"]}). No query string yields {} —
matches the empty-POST-body semantics.
HEAD returns 200 empty so Slack/Discord link previews don't fire the workflow
AND don't flag the URL as broken. Other verbs (PUT/PATCH/DELETE/OPTIONS) still
return 405 JSON with Allow: POST, GET.
Frontend WebhookUrlHint now advertises both verbs and drops the POST prefix
from the displayed URL so the raw URL is copy-and-clickable.
6 new xUnit tests cover: GET-unknown-404, GET-with-query, GET-repeated-keys,
GET-empty-query, HEAD-200, PUT-still-405 (updated Allow header).
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