feat(config): advertise a concurrency limit to clients

Self-hosted upstreams serve few requests at once. A client that opens
many requests overloads them.

The new maxConcurrency option sends X-Concurrency-Limit on every
response. Clients that honour the header cap their own in-flight
requests. The default of 0 sends no header, so behaviour does not
change.

concurrency_header_test.go

@@ -0,0 +1,55 @@
+package traefikllmgateway
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+func newConcurrencyTestGateway(t *testing.T, maxConcurrency int) (http.Handler, error) {
+	t.Helper()
+	cfg := CreateConfig()
+	cfg.MaxConcurrency = maxConcurrency
+	cfg.Providers = map[string]*ProviderConfig{
+		"openai": {Type: "openai", APIKey: "sk-test", Models: []string{"gpt-test"}},
+	}
+	cfg.Groups = map[string]*GroupConfig{"default": {}}
+	cfg.Users = &UsersConfig{Inline: []*UserConfig{{Name: "tester", Group: "default", APIKey: "sk-user"}}}
+	return New(context.Background(), http.HandlerFunc(func(http.ResponseWriter, *http.Request) {}), cfg, "llmgw")
+}
+
+func TestMaxConcurrency_AdvertisedOnResponses(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		name   string
+		header string
+		max    int
+	}{
+		{"configured limit is advertised", "3", 3},
+		{"zero sends no header", "", 0},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			h, err := newConcurrencyTestGateway(t, tc.max)
+			if err != nil {
+				t.Fatalf("New: %v", err)
+			}
+			req := httptest.NewRequest(http.MethodGet, "/v1/models", nil)
+			req.Header.Set("Authorization", "Bearer sk-user")
+			rec := httptest.NewRecorder()
+			h.ServeHTTP(rec, req)
+			if got := rec.Header().Get("X-Concurrency-Limit"); got != tc.header {
+				t.Fatalf("X-Concurrency-Limit = %q, want %q", got, tc.header)
+			}
+		})
+	}
+}
+
+func TestMaxConcurrency_NegativeRejected(t *testing.T) {
+	t.Parallel()
+	if _, err := newConcurrencyTestGateway(t, -1); err == nil {
+		t.Fatal("New accepted a negative maxConcurrency")
+	}
+}