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/delivery/engine_test.go

@@ -27,6 +27,9 @@ import (
 	"sneak.berlin/go/webhooker/internal/delivery"
 )
 
+// testContentType is the event content type used in tests.
+const testContentType = "application/json"
+
 func testWebhookDB(t *testing.T) *gorm.DB {
 	t.Helper()
 
@@ -94,10 +97,10 @@ func seedEvent(
 	event := database.Event{
 		WebhookID:    uuid.New().String(),
 		EntrypointID: uuid.New().String(),
-		Method:       "POST",
+		Method:       http.MethodPost,
 		Headers:      `{"Content-Type":["application/json"]}`,
 		Body:         body,
-		ContentType:  "application/json",
+		ContentType:  testContentType,
 	}
 
 	require.NoError(t, db.Create(&event).Error)
@@ -1113,10 +1116,10 @@ func TestDoHTTPRequest_ForwardsHeaders(t *testing.T) {
 	}
 
 	event := &database.Event{
-		Method:      "POST",
+		Method:      http.MethodPost,
 		Headers:     `{"X-Custom":["value1"],"Content-Type":["application/json"]}`,
 		Body:        `{"test":true}`,
-		ContentType: "application/json",
+		ContentType: testContentType,
 	}
 
 	statusCode, _, _, err := e.ExportDoHTTPRequest(
@@ -1138,7 +1141,7 @@ func TestDoHTTPRequest_ForwardsHeaders(t *testing.T) {
 	)
 
 	assert.Equal(t,
-		"application/json",
+		testContentType,
 		receivedHeaders.Get("Content-Type"),
 	)
 
@@ -1297,8 +1300,8 @@ func TestFormatSlackMessage_JSONBody(t *testing.T) {
 	t.Parallel()
 
 	event := &database.Event{
-		Method:      "POST",
-		ContentType: "application/json",
+		Method:      http.MethodPost,
+		ContentType: testContentType,
 		Body: `{"action":"push",` +
 			`"repo":"test/repo",` +
 			`"ref":"refs/heads/main"}`,
@@ -1323,7 +1326,7 @@ func TestFormatSlackMessage_NonJSONBody(t *testing.T) {
 	t.Parallel()
 
 	event := &database.Event{
-		Method:      "POST",
+		Method:      http.MethodPost,
 		ContentType: "text/plain",
 		Body:        "hello world plain text",
 	}
@@ -1346,8 +1349,8 @@ func TestFormatSlackMessage_EmptyBody(t *testing.T) {
 	t.Parallel()
 
 	event := &database.Event{
-		Method:      "POST",
-		ContentType: "application/json",
+		Method:      http.MethodPost,
+		ContentType: testContentType,
 		Body:        "",
 	}
 	event.CreatedAt = time.Date(
@@ -1375,8 +1378,8 @@ func TestFormatSlackMessage_LargeJSONTruncated(
 	require.NoError(t, err)
 
 	event := &database.Event{
-		Method:      "POST",
-		ContentType: "application/json",
+		Method:      http.MethodPost,
+		ContentType: testContentType,
 		Body:        string(largeJSON),
 	}
 	event.CreatedAt = time.Date(
@@ -1705,7 +1708,7 @@ func assertLogLineComplete(
 		"log line must contain the webhook id",
 	)
 
-	assert.Contains(t, out, "application/json",
+	assert.Contains(t, out, testContentType,
 		"log line must contain the content type",
 	)
 }