build: update Go to 1.27.1 and modernize the codebase (#2798)

Toolchain and CI:
- Bump go directive from 1.26.8 to 1.27.1 (go.mod)
- Use golang:1.27-alpine builder images in all Dockerfiles
- Bump golangci-lint to v2.13.2 (first release with go1.27 support)
- Add CI gate: `go fix -diff` fails when outdated Go patterns are
  detected (locally: `make check-modernize`)

Note: darwin binaries now require macOS 13 or later.

Lint fixes required by golangci-lint v2.13.2:
- goconst: ignore tests (all 436 findings were test-only; goconst
  got stricter since v2.12 and this option was added for it)
- openai.go: keep deprecated MaxTokens deliberately with a nolint
  rationale (max_tokens is the only form universally supported by
  OpenAI-compatible backends like One-API, LiteLLM, Ollama shim)
- state.go: drop always-nil flags param from appendChartVersionFlags
  (renamed to chartVersionFlags, unparam)

Modernization (go fix ./..., 62 files):
- interface{} -> any, maps.Copy, strings.SplitSeq, range-over-int,
  builtin min/max, slices.Contains/ContainsFunc/Sort, WaitGroup.Go,
  reflect.Type.Fields(), new(expr)
- exit_error.go: strings.Builder + fmt.Fprintf instead of string
  concatenation and WriteString(fmt.Sprintf(...)) (QF1012)
- chart_dependency.go: strings.CutLast for OCI dependency helpers

Signed-off-by: yxxhero <aiopsclub@163.com>
Co-authored-by: Claude <noreply@anthropic.com>

pkg/state/chart_dependency.go

@@ -260,8 +260,8 @@ func chartDependenciesAlias(namespace, releaseName string) string {
 // the chart path when helm processes OCI references during dependency update.
 // See issue #954.
 func ociDependencyChartName(chart string) string {
-	if idx := strings.LastIndex(chart, "/"); idx >= 0 {
-		return chart[idx+1:]
+	if _, name, found := strings.CutLast(chart, "/"); found {
+		return name
 	}
 	return chart
 }
@@ -273,10 +273,11 @@ func ociDependencyChartName(chart string) string {
 //	chart="path_with_underscores/example", baseURL="oci://registry.example.com"
 //	→ "oci://registry.example.com/path_with_underscores"
 func ociDependencyRepoURL(chart, ociBaseURL string) string {
-	if idx := strings.LastIndex(chart, "/"); idx >= 0 {
-		return strings.TrimSuffix(ociBaseURL, "/") + "/" + chart[:idx]
+	pathPrefix, _, found := strings.CutLast(chart, "/")
+	if !found {
+		return ociBaseURL
 	}
-	return ociBaseURL
+	return strings.TrimSuffix(ociBaseURL, "/") + "/" + pathPrefix
 }
 
 func getUnresolvedDependenciess(st *HelmState) (string, *UnresolvedDependencies) {