✨ 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/bdd/testserver/server.go

@@ -100,5 +100,8 @@ func createTestConfig(port int) *config.Config {
 		Telemetry: config.TelemetryConfig{
 			Enabled: false,
 		},
+		API: config.APIConfig{
+			V2Enabled: true, // Enable v2 for testing
+		},
 	}
 }