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- Add CI gate: `go fix -diff` fails when outdated Go patterns are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/agent/doctor/report_test.go

@@ -224,7 +224,7 @@ func TestReportJSON_FieldOrderIsHumanReadable(t *testing.T) {
 	// Cheap way: scan line-by-line, the first 7 non-space chars of each line
 	// are `"key":`.
 	keys := []string{}
-	for _, line := range strings.Split(out, "\n") {
+	for line := range strings.SplitSeq(out, "\n") {
 		line = strings.TrimSpace(line)
 		if !strings.HasPrefix(line, `"`) {
 			continue

pkg/agent/llm/openai.go

@@ -50,7 +50,10 @@ func (o *openaiClient) Analyze(ctx goContext.Context, diff string, extras Analyz
 	req := openai.ChatCompletionRequest{
 		Model:       o.cfg.Model,
 		Temperature: o.cfg.Temperature,
-		MaxTokens:   o.cfg.MaxTokens,
+		// SA1019: keep the deprecated MaxTokens field deliberately: `max_tokens` is the
+		// only form universally accepted by OpenAI-compatible backends (One-API,
+		// LiteLLM, Ollama shim); MaxCompletionTokens is not supported by all of them.
+		MaxTokens: o.cfg.MaxTokens, //nolint:staticcheck // SA1019
 		ResponseFormat: &openai.ChatCompletionResponseFormat{
 			Type: openai.ChatCompletionResponseFormatTypeJSONObject,
 		},
@@ -166,8 +169,8 @@ func stripJSONCodeFence(s string) string {
 	// Case 2: fenced JSON.
 	if strings.HasPrefix(s, "```") {
 		rest := s[3:]
-		if nl := strings.IndexByte(rest, '\n'); nl >= 0 {
-			s = rest[nl+1:]
+		if _, after, ok := strings.Cut(rest, "\n"); ok {
+			s = after
 		} else {
 			s = rest
 		}

pkg/agent/llm/types_test.go

@@ -141,7 +141,7 @@ func TestMockClient_Analyze_PropagatesErr(t *testing.T) {
 func TestMockClient_ConcurrentSafe(t *testing.T) {
 	c := NewMockClient(Analysis{Summary: "ok"})
 	var wg sync.WaitGroup
-	for i := 0; i < 20; i++ {
+	for range 20 {
 		wg.Add(2)
 		go func() {
 			defer wg.Done()