TUN-9858: Remove proxy-dns feature from cloudflared

Remove the DNS over HTTPS (DoH) proxy feature built on CoreDNS due to
security vulnerabilities (GO-2025-3942, GO-2026-4289).

This removes:
- Standalone proxy-dns command (cloudflared proxy-dns)
- Tunnel subcommand (cloudflared tunnel proxy-dns)
- Proxy-dns flags for tunnel run (--proxy-dns, --proxy-dns-port, etc.)
- Config file resolver section support
- tunneldns/ package (CoreDNS-based implementation)
- Related component tests

BREAKING CHANGE: The proxy-dns feature is no longer available.
Users should migrate to alternative DNS over HTTPS solutions.

config/model.go

@@ -4,9 +4,6 @@ import (
 	"crypto/sha256"
 	"fmt"
 	"io"
-	"strings"
-
-	"github.com/cloudflare/cloudflared/tunneldns"
 )
 
 // Forwarder represents a client side listener to forward traffic to the edge
@@ -26,23 +23,13 @@ type Tunnel struct {
 	ProtocolType string `json:"type"`
 }
 
-// DNSResolver represents a client side DNS resolver
-type DNSResolver struct {
-	Enabled                bool     `json:"enabled"`
-	Address                string   `json:"address,omitempty"`
-	Port                   uint16   `json:"port,omitempty"`
-	Upstreams              []string `json:"upstreams,omitempty"`
-	Bootstraps             []string `json:"bootstraps,omitempty"`
-	MaxUpstreamConnections int      `json:"max_upstream_connections,omitempty"`
-}
-
-// Root is the base options to configure the service
+// Root is the base options to configure the service.
 type Root struct {
 	LogDirectory string      `json:"log_directory" yaml:"logDirectory,omitempty"`
 	LogLevel     string      `json:"log_level" yaml:"logLevel,omitempty"`
 	Forwarders   []Forwarder `json:"forwarders,omitempty" yaml:"forwarders,omitempty"`
 	Tunnels      []Tunnel    `json:"tunnels,omitempty" yaml:"tunnels,omitempty"`
-	Resolver     DNSResolver `json:"resolver,omitempty" yaml:"resolver,omitempty"`
+	// `resolver` key is reserved for a removed feature (proxy-dns) and should not be used.
 }
 
 // Hash returns the computed values to see if the forwarder values change
@@ -55,60 +42,3 @@ func (f *Forwarder) Hash() string {
 	_, _ = io.WriteString(h, f.Destination)
 	return fmt.Sprintf("%x", h.Sum(nil))
 }
-
-// Hash returns the computed values to see if the forwarder values change
-func (r *DNSResolver) Hash() string {
-	h := sha256.New()
-	_, _ = io.WriteString(h, r.Address)
-	_, _ = io.WriteString(h, strings.Join(r.Bootstraps, ","))
-	_, _ = io.WriteString(h, strings.Join(r.Upstreams, ","))
-	_, _ = io.WriteString(h, fmt.Sprintf("%d", r.Port))
-	_, _ = io.WriteString(h, fmt.Sprintf("%d", r.MaxUpstreamConnections))
-	_, _ = io.WriteString(h, fmt.Sprintf("%v", r.Enabled))
-	return fmt.Sprintf("%x", h.Sum(nil))
-}
-
-// EnabledOrDefault returns the enabled property
-func (r *DNSResolver) EnabledOrDefault() bool {
-	return r.Enabled
-}
-
-// AddressOrDefault returns the address or returns the default if empty
-func (r *DNSResolver) AddressOrDefault() string {
-	if r.Address != "" {
-		return r.Address
-	}
-	return "localhost"
-}
-
-// PortOrDefault return the port or returns the default if 0
-func (r *DNSResolver) PortOrDefault() uint16 {
-	if r.Port > 0 {
-		return r.Port
-	}
-	return 53
-}
-
-// UpstreamsOrDefault returns the upstreams or returns the default if empty
-func (r *DNSResolver) UpstreamsOrDefault() []string {
-	if len(r.Upstreams) > 0 {
-		return r.Upstreams
-	}
-	return []string{"https://1.1.1.1/dns-query", "https://1.0.0.1/dns-query"}
-}
-
-// BootstrapsOrDefault returns the bootstraps or returns the default if empty
-func (r *DNSResolver) BootstrapsOrDefault() []string {
-	if len(r.Bootstraps) > 0 {
-		return r.Bootstraps
-	}
-	return []string{"https://162.159.36.1/dns-query", "https://162.159.46.1/dns-query", "https://[2606:4700:4700::1111]/dns-query", "https://[2606:4700:4700::1001]/dns-query"}
-}
-
-// MaxUpstreamConnectionsOrDefault return the max upstream connections or returns the default if negative
-func (r *DNSResolver) MaxUpstreamConnectionsOrDefault() int {
-	if r.MaxUpstreamConnections >= 0 {
-		return r.MaxUpstreamConnections
-	}
-	return tunneldns.MaxUpstreamConnsDefault
-}