Add server WriteJSON and WriteError response helpers

Eliminates repeated marshal-set-header-write boilerplate in handlers.
WriteError produces consistent {"error": "..."} JSON responses.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/respond_test.go

@@ -0,0 +1,72 @@
+package server_test
+
+import (
+	"encoding/json"
+	"net/http/httptest"
+	"testing"
+
+	"git.codelab.vc/pkg/httpx/server"
+)
+
+func TestWriteJSON(t *testing.T) {
+	t.Run("writes JSON with status and content type", func(t *testing.T) {
+		w := httptest.NewRecorder()
+
+		type resp struct {
+			ID   int    `json:"id"`
+			Name string `json:"name"`
+		}
+
+		err := server.WriteJSON(w, 201, resp{ID: 1, Name: "Alice"})
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+
+		if w.Code != 201 {
+			t.Fatalf("got status %d, want %d", w.Code, 201)
+		}
+		if ct := w.Header().Get("Content-Type"); ct != "application/json" {
+			t.Fatalf("Content-Type = %q, want %q", ct, "application/json")
+		}
+
+		var decoded resp
+		if err := json.Unmarshal(w.Body.Bytes(), &decoded); err != nil {
+			t.Fatalf("unmarshal: %v", err)
+		}
+		if decoded.ID != 1 || decoded.Name != "Alice" {
+			t.Fatalf("got %+v, want {ID:1 Name:Alice}", decoded)
+		}
+	})
+
+	t.Run("returns error for unmarshalable input", func(t *testing.T) {
+		w := httptest.NewRecorder()
+
+		err := server.WriteJSON(w, 200, make(chan int))
+		if err == nil {
+			t.Fatal("expected error for channel type")
+		}
+	})
+}
+
+func TestWriteError(t *testing.T) {
+	w := httptest.NewRecorder()
+
+	err := server.WriteError(w, 404, "not found")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	if w.Code != 404 {
+		t.Fatalf("got status %d, want %d", w.Code, 404)
+	}
+
+	var body struct {
+		Error string `json:"error"`
+	}
+	if err := json.Unmarshal(w.Body.Bytes(), &body); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if body.Error != "not found" {
+		t.Fatalf("error = %q, want %q", body.Error, "not found")
+	}
+}