Update golangci-lint to v2.12.2 with canonical config

Bump the golangci-lint Docker image pin in Dockerfile and the
release-archive sha256 pins in script/bootstrap from 2.11.3 to
2.12.2, and replace .golangci.yml with the canonical config. The
canonical config moves lll/funlen/cyclop/dupl settings from the
top-level linters-settings key (ignored by the v2 schema) to
linters.settings, so those thresholds now actually apply.

Fix all findings the newly applied thresholds surfaced:

- lll: wrap or shorten seven over-length lines (struct tag
  comments, test logger construction, a func signature, and a
  nosec comment)
- goconst: use http.MethodPost/http.MethodPut and new shared
  constants for repeated test strings; add tmplKeyError and
  tmplKeyWebhook constants for template data keys in handlers
- dupl: merge buildHTTPTargetConfig and buildSlackTargetConfig
  into a parameterized buildURLTargetConfig; drop the duplicate
  iWebhookDB test helper in favor of testWebhookDB; extract
  shared helpers in middleware and session tests

internal/middleware/middleware_test.go

@@ -484,8 +484,13 @@ func metricsAuthMiddleware(
 	return middleware.NewForTest(log, cfg, sessManager)
 }
 
-func TestMetricsAuth_ValidCredentials(t *testing.T) {
-	t.Parallel()
+// runMetricsAuthRequest sends a GET /metrics request with the
+// given basic-auth password through MetricsAuth and reports
+// whether the wrapped handler ran plus the recorded response.
+func runMetricsAuthRequest(
+	t *testing.T, password string,
+) (bool, *httptest.ResponseRecorder) {
+	t.Helper()
 
 	m := metricsAuthMiddleware(t)
 
@@ -503,12 +508,20 @@ func TestMetricsAuth_ValidCredentials(t *testing.T) {
 		context.Background(),
 		http.MethodGet, "/metrics", nil,
 	)
-	req.SetBasicAuth("admin", "secret")
+	req.SetBasicAuth("admin", password)
 
 	w := httptest.NewRecorder()
 
 	handler.ServeHTTP(w, req)
 
+	return called, w
+}
+
+func TestMetricsAuth_ValidCredentials(t *testing.T) {
+	t.Parallel()
+
+	called, w := runMetricsAuthRequest(t, "secret")
+
 	assert.True(
 		t, called,
 		"handler should be called with valid basic auth",
@@ -519,27 +532,7 @@ func TestMetricsAuth_ValidCredentials(t *testing.T) {
 func TestMetricsAuth_InvalidCredentials(t *testing.T) {
 	t.Parallel()
 
-	m := metricsAuthMiddleware(t)
-
-	var called bool
-
-	handler := m.MetricsAuth()(http.HandlerFunc(
-		func(w http.ResponseWriter, _ *http.Request) {
-			called = true
-
-			w.WriteHeader(http.StatusOK)
-		},
-	))
-
-	req := httptest.NewRequestWithContext(
-		context.Background(),
-		http.MethodGet, "/metrics", nil,
-	)
-	req.SetBasicAuth("admin", "wrong-password")
-
-	w := httptest.NewRecorder()
-
-	handler.ServeHTTP(w, req)
+	called, w := runMetricsAuthRequest(t, "wrong-password")
 
 	assert.False(
 		t, called,