encode: prioritize zstd and br over gzip in content negotiation (#7772)

* fix(encode): prioritize zstd and br over gzip in content negotiation

* test(encode): update unit tests to reflect new default priority ties

* fix(encode): move default preferences to dynamic encode handler and restore generic negotiation helper

* test(encode): call real Provision function in served-response test

* test(encode): rename served-response test to TestServeHTTPDefaultEncodingPreference

* refactor(encode): use slices.SortStableFunc and httptest.NewRecorder as recommended

* refactor(encode): simplify sorting with cmp.Compare and check request error in test

* test(encode): fix variable redeclaration in TestServeHTTPDefaultEncodingPreference

Fix 'no new variables on left side of :=' error by changing 'err :=' to 'err ='
on line 347, since err was already declared on line 332.

This fixes the build failure in the encode module tests.

modules/caddyhttp/encode/encode.go

@@ -20,12 +20,12 @@
 package encode
 
 import (
+	"cmp"
 	"fmt"
 	"io"
 	"math"
 	"net/http"
 	"slices"
-	"sort"
 	"strconv"
 	"strings"
 	"sync"
@@ -127,6 +127,14 @@ func (enc *Encode) Provision(ctx caddy.Context) error {
 		}
 	}
 
+	if len(enc.Prefer) == 0 {
+		for _, encName := range []string{"zstd", "br", "gzip"} {
+			if _, ok := enc.writerPools[encName]; ok {
+				enc.Prefer = append(enc.Prefer, encName)
+			}
+		}
+	}
+
 	return nil
 }
 
@@ -538,11 +546,11 @@ func AcceptedEncodings(r *http.Request, preferredOrder []string) []string {
 	}
 
 	// sort preferences by descending q-factor first, then by preferOrder
-	sort.Slice(prefs, func(i, j int) bool {
-		if math.Abs(prefs[i].q-prefs[j].q) < 0.00001 {
-			return prefs[i].preferOrder > prefs[j].preferOrder
+	slices.SortStableFunc(prefs, func(a, b encodingPreference) int {
+		if math.Abs(a.q-b.q) < 0.00001 {
+			return cmp.Compare(b.preferOrder, a.preferOrder)
 		}
-		return prefs[i].q > prefs[j].q
+		return cmp.Compare(b.q, a.q)
 	})
 
 	prefEncNames := make([]string, len(prefs))