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/session/session_test.go

@@ -173,8 +173,18 @@ func TestSetUser_SetsAllFields(t *testing.T) {
 	)
 }
 
-func TestGetUserID(t *testing.T) {
-	t.Parallel()
+// testSessionGetter exercises a session string getter before and
+// after SetUser: it must report false with an empty value on a
+// fresh session, then true with the expected value once
+// SetUser(sess, "user-xyz", "bob") has run.
+func testSessionGetter(
+	t *testing.T,
+	get func(
+		*session.Session, *sessions.Session,
+	) (string, bool),
+	expected string,
+) {
+	t.Helper()
 
 	s := testSession(t)
 
@@ -185,44 +195,46 @@ func TestGetUserID(t *testing.T) {
 	require.NoError(t, err)
 
 	// Before setting user
-	userID, ok := s.GetUserID(sess)
+	val, ok := get(s, sess)
 	assert.False(
-		t, ok, "should return false when no user ID is set",
+		t, ok, "should return false before SetUser",
 	)
-	assert.Empty(t, userID)
+	assert.Empty(t, val)
 
 	// After setting user
 	s.SetUser(sess, "user-xyz", "bob")
 
-	userID, ok = s.GetUserID(sess)
+	val, ok = get(s, sess)
 	assert.True(t, ok)
-	assert.Equal(t, "user-xyz", userID)
+	assert.Equal(t, expected, val)
+}
+
+func TestGetUserID(t *testing.T) {
+	t.Parallel()
+
+	testSessionGetter(
+		t,
+		func(
+			s *session.Session, sess *sessions.Session,
+		) (string, bool) {
+			return s.GetUserID(sess)
+		},
+		"user-xyz",
+	)
 }
 
 func TestGetUsername(t *testing.T) {
 	t.Parallel()
 
-	s := testSession(t)
-
-	req := httptest.NewRequestWithContext(
-		context.Background(), http.MethodGet, "/", nil)
-
-	sess, err := s.Get(req)
-	require.NoError(t, err)
-
-	// Before setting user
-	username, ok := s.GetUsername(sess)
-	assert.False(
-		t, ok, "should return false when no username is set",
+	testSessionGetter(
+		t,
+		func(
+			s *session.Session, sess *sessions.Session,
+		) (string, bool) {
+			return s.GetUsername(sess)
+		},
+		"bob",
 	)
-	assert.Empty(t, username)
-
-	// After setting user
-	s.SetUser(sess, "user-xyz", "bob")
-
-	username, ok = s.GetUsername(sess)
-	assert.True(t, ok)
-	assert.Equal(t, "bob", username)
 }
 
 // --- IsAuthenticated Tests ---