[BUG] Generic webhook does not accept the methods, headers, or query parameters its Setup Instructions promise #6888

Describe the bug

The generic webhook trigger's Setup Instructions promise three things the endpoint does not do:

The trigger also declares outputs: {}, so the reference dropdown in the editor offers no completions for the request metadata and users have to type paths by hand.

A concrete consequence: a webhook URL sent in an email cannot be triggered by clicking the link, because a plain browser GET used to answer 405. (GET delivery is the one part that has since been addressed.)

To Reproduce

1. Create a workflow with a Webhook trigger, provider "Generic".
2. Read the Setup Instructions shown in the trigger's config panel.
3. curl -X PUT "$WEBHOOK_URL?id=42" -H 'X-Custom: hello' -d '{}' → 405 Method Not Allowed.
4. curl -X POST "$WEBHOOK_URL?id=42" -H 'X-Custom: hello' -d '{"a":1}' → 200, but the workflow input contains only {"a":1}: no headers, no query, no method.

Expected behavior

The endpoint should behave the way the Setup Instructions describe: accept the documented HTTP methods for the generic provider, and expose the request method, query parameters, and request headers to the workflow — with credential-bearing headers withheld so they don't end up in execution logs. Providers other than generic should keep answering 405 for non-POST deliveries.

Additional context

The Setup Instructions text was introduced in #903; the route at that commit already accepted only GET and POST, so this is not a regression — the documentation was ahead of the implementation from the start. Happy to open a PR against staging; I have the change implemented and tested locally.