[management] Return the reason when custom domain validation fails (#8122)

* [management] Return the reason when custom domain validation fails

The validate endpoint returned an empty 202 and ran the check in a
goroutine, so a failed verification only reached the management log.
Run the check in the request and return 204 or 412 with a classified
reason. Resolver errors are not forwarded.

* [management] Address review feedback on custom domain validation

Pass the request context to the lookup and store calls now that the
check runs in the request, compare the self-resolved name
case-insensitively, document the 412 like the other entries, and
check the returned error in the existing tests.

* [management] Match custom domain CNAME targets case-insensitively

DNS names are case-insensitive, so a CNAME pointing at the cluster in
different casing should validate instead of failing as a mismatch.

* [management] Add validation timeout

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Co-authored-by: DevBot NetBird <91386968+netbirddev@users.noreply.github.com>

management/internals/modules/reverseproxy/domain/manager/api.go

@@ -134,7 +134,10 @@ func (h *handler) triggerCustomDomainValidation(w http.ResponseWriter, r *http.R
 		return
 	}
 
-	go h.manager.ValidateDomain(r.Context(), userAuth.AccountId, userAuth.UserId, domainID)
+	if err := h.manager.ValidateDomain(r.Context(), userAuth.AccountId, userAuth.UserId, domainID); err != nil {
+		util.WriteError(r.Context(), err, w)
+		return
+	}
 
-	w.WriteHeader(http.StatusAccepted)
+	w.WriteHeader(http.StatusNoContent)
 }