feat(ednsdiag): support custom DoH endpoint via --url

Allow overriding the provider preset with an explicit HTTPS DoH URL,
including validation that custom endpoints apply only to DoH queries.

.agents/skills/encrypted-dns-skill/internal/edns/model.go

@@ -6,6 +6,7 @@ type QueryOptions struct {
 	Protocol   string
 	Provider   string
 	Method     string
+	EndpointURL string
 }
 
 type Result struct {

.agents/skills/encrypted-dns-skill/internal/edns/query.go

@@ -3,6 +3,7 @@ package edns
 import (
 	"context"
 	"fmt"
+	"net/url"
 )
 
 func Query(ctx context.Context, options QueryOptions) Result {
@@ -29,6 +30,18 @@ func Query(ctx context.Context, options QueryOptions) Result {
 		result.Error = &ErrorInfo{Class: "input", Message: err.Error()}
 		return result
 	}
+	if options.EndpointURL != "" {
+		if options.Protocol != "doh" {
+			result.Error = &ErrorInfo{Class: "input", Message: "custom --url applies only to DoH"}
+			return result
+		}
+		parsed, err := url.Parse(options.EndpointURL)
+		if err != nil || parsed.Scheme != "https" || parsed.Hostname() == "" {
+			result.Error = &ErrorInfo{Class: "input", Message: fmt.Sprintf("invalid DoH endpoint URL %q", options.EndpointURL)}
+			return result
+		}
+		provider = Provider{ID: "custom", Profile: "custom", DoHURL: options.EndpointURL}
+	}
 	result.Resolver = ResolverInfo{Provider: provider.ID, Profile: provider.Profile}
 
 	var response []byte