feat(applog): add JSON logger and context helpers

backend/internal/applog/applog_test.go

@@ -0,0 +1,54 @@
+package applog
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"log/slog"
+	"strings"
+	"testing"
+)
+
+func TestNewLogger_FiltersBelowConfiguredLevel(t *testing.T) {
+	var buf bytes.Buffer
+	logger := NewLogger("warn", &buf)
+
+	logger.Info("should be dropped")
+	if buf.Len() != 0 {
+		t.Fatalf("expected no output for an Info message under a warn-level logger, got %q", buf.String())
+	}
+
+	logger.Warn("should appear")
+	if !strings.Contains(buf.String(), "should appear") {
+		t.Fatalf("expected the Warn message in output, got %q", buf.String())
+	}
+}
+
+func TestNewLogger_WritesValidJSON(t *testing.T) {
+	var buf bytes.Buffer
+	logger := NewLogger("info", &buf)
+	logger.Info("hello", "key", "value")
+
+	var decoded map[string]any
+	if err := json.Unmarshal(buf.Bytes(), &decoded); err != nil {
+		t.Fatalf("output is not valid JSON: %v (%q)", err, buf.String())
+	}
+	if decoded["msg"] != "hello" || decoded["key"] != "value" {
+		t.Errorf("decoded = %+v, want msg=hello key=value", decoded)
+	}
+}
+
+func TestWithLogger_FromContext_RoundTrip(t *testing.T) {
+	logger := slog.New(slog.NewJSONHandler(&bytes.Buffer{}, nil))
+	ctx := WithLogger(context.Background(), logger)
+
+	if got := FromContext(ctx); got != logger {
+		t.Errorf("FromContext returned a different logger than what was stashed")
+	}
+}
+
+func TestFromContext_DefaultsWhenNoneSet(t *testing.T) {
+	if got := FromContext(context.Background()); got == nil {
+		t.Fatal("FromContext on a bare context returned nil, want slog.Default()")
+	}
+}