✨ feat: implement API v2 with feature flag control

- Added /api/v2/greet POST endpoint with JSON request/response

- Implemented ServiceV2 with Hello my friend <name>! greeting format

- Added api.v2_enabled feature flag (default: false)

- Extended BDD tests to cover v2 scenarios

- Maintained full backward compatibility with v1 API

- Added DLC_API_V2_ENABLED environment variable support

- Created ADR 0010-api-v2-feature-flag.md

- Updated configuration system to support API versioning

pkg/greet/greet_v2_test.go

@@ -0,0 +1,28 @@
+package greet
+
+import (
+	"context"
+	"testing"
+)
+
+func TestServiceV2_GreetV2(t *testing.T) {
+	service := NewServiceV2()
+	tests := []struct {
+		name     string
+		expected string
+	}{
+		{"", "Hello my friend!"},
+		{"John", "Hello my friend John!"},
+		{"Alice", "Hello my friend Alice!"},
+		{"  ", "Hello my friend   !"}, // spaces are not considered empty
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := service.GreetV2(context.Background(), tt.name)
+			if result != tt.expected {
+				t.Errorf("GreetV2(%q) = %q, want %q", tt.name, result, tt.expected)
+			}
+		})
+	}
+}