Summary

Fixes #6999 — sending on a nil channel in RequestHandler blocks the HTTP worker goroutine forever, enabling a denial-of-service condition.

Root cause: webhook.requestQ[sourceDefName] returns nil when the source has not been registered (e.g. webhookV2HandlerEnabled=false and no prior Register call). Go's nil channel send blocks indefinitely.

Changes:

• Nil-channel guard — check requestQ == nil after the map lookup and return HTTP 503 service unavailable immediately instead of blocking.
• Shutdown-aware enqueue — wrap the channel send in a select with r.Context().Done() (client disconnect / deadline) and webhook.ctx.Done() (server shutdown). The RLock is held across the select to prevent a concurrent Shutdown from closing the channel mid-send.
• Buffered done channel (size 1) — if RequestHandler exits early due to context cancellation, the batcher goroutine can still complete its send without leaking.
• Context guard on resp := <-done — also wraps the response-wait in a select with r.Context().Done() so a slow transformer doesn't permanently hold a worker after the client has gone.
• Store shutdown context — HandleT.ctx added and populated in Setup so RequestHandler can reference it.

Test plan

• TestWebhookRequestHandlerSourceNotRegistered — V1 mode, no Register call → asserts HTTP 503 returned immediately, no block
• TestWebhookRequestHandlerRequestContextCancellation — blocking adapter + 100 ms request deadline → asserts handler returns promptly
• All existing webhook unit tests pass (go test ./gateway/webhook/... -race -count=1)
• Manual: run with webhookV2HandlerEnabled=false and send a request for an unregistered source — expect immediate 503