build: update golangci-lint to v2.12.2 with canonical config

Add the canonical .golangci.yml (v2 schema, all linters enabled with a
small documented disable list) and pin the Dockerfile lint stage to
golangci/golangci-lint:v2.12.2 by tag and digest, replacing the old
v1.64.8 digest-only pin.

Fix all findings surfaced by the v1 to v2 jump without changing any
exported signatures or behavior:

- add package and exported-symbol doc comments (revive)
- rename unused handler parameters to underscore (revive)
- check or explicitly discard error returns (errcheck, errchkjson)
- wrap errors with %w instead of %v (err113)
- use http.NewRequestWithContext instead of http.Post (noctx)
- replace fmt.Println with fmt.Fprintln(os.Stdout, ...) (forbidigo)
- name magic numbers as constants (mnd)
- add explicit slog.LevelDebug case (exhaustive)
- interface{} to any (modernize)
- move tests to the simplelog_test package (testpackage) and add
  t.Parallel() (paralleltest)
- move NewWebhookHandler above its methods (funcorder)
- whitespace, line-length, and blank-line fixes (wsl_v5, whitespace,
  nlreturn, lll, embeddedstructfieldcheck)
- nolint with justification for the intentional init/global design
  (gochecknoinits, gochecknoglobals) and interface-returning
  constructor (ireturn)

webhook_handler.go

@@ -10,38 +10,64 @@ import (
 	"net/url"
 )
 
+// WebhookHandler POSTs each log record as JSON to a configured webhook
+// URL.
 type WebhookHandler struct {
 	webhookURL string
 }
 
-func (w *WebhookHandler) Enabled(ctx context.Context, level slog.Level) bool {
-	return true
-}
-
-func (w *WebhookHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
-	return w
-}
-
-func (w *WebhookHandler) WithGroup(name string) slog.Handler {
-	return w
-}
-
+// NewWebhookHandler returns a WebhookHandler that delivers records to
+// the given URL, validating the URL first.
 func NewWebhookHandler(webhookURL string) (*WebhookHandler, error) {
-	if _, err := url.ParseRequestURI(webhookURL); err != nil {
-		return nil, fmt.Errorf("invalid webhook URL: %v", err)
+	_, err := url.ParseRequestURI(webhookURL)
+	if err != nil {
+		return nil, fmt.Errorf("invalid webhook URL: %w", err)
 	}
+
 	return &WebhookHandler{webhookURL: webhookURL}, nil
 }
 
+// Enabled reports whether the handler processes records at the given
+// level; it always returns true.
+func (w *WebhookHandler) Enabled(_ context.Context, _ slog.Level) bool {
+	return true
+}
+
+// WithAttrs returns the handler unchanged; attributes are not rendered.
+func (w *WebhookHandler) WithAttrs(_ []slog.Attr) slog.Handler {
+	return w
+}
+
+// WithGroup returns the handler unchanged; groups are not rendered.
+func (w *WebhookHandler) WithGroup(_ string) slog.Handler {
+	return w
+}
+
+// Handle marshals the record to JSON and POSTs it to the webhook URL.
 func (w *WebhookHandler) Handle(ctx context.Context, record slog.Record) error {
 	jsonData, err := json.Marshal(record)
 	if err != nil {
-		return fmt.Errorf("error marshaling event: %v", err)
+		return fmt.Errorf("error marshaling event: %w", err)
 	}
-	response, err := http.Post(w.webhookURL, "application/json", bytes.NewBuffer(jsonData))
+
+	request, err := http.NewRequestWithContext(
+		ctx,
+		http.MethodPost,
+		w.webhookURL,
+		bytes.NewReader(jsonData),
+	)
+	if err != nil {
+		return fmt.Errorf("error creating webhook request: %w", err)
+	}
+
+	request.Header.Set("Content-Type", "application/json")
+
+	response, err := http.DefaultClient.Do(request)
 	if err != nil {
 		return err
 	}
-	defer response.Body.Close()
+
+	defer func() { _ = response.Body.Close() }()
+
 	return nil
 }