feat: implement BDD testing with Godog

Implement comprehensive BDD testing framework using Godog:
- Added feature files for greet and health endpoints
- Created test server that runs on port 9191
- Implemented step definitions using Godog's exact patterns
- Fixed undefined step warnings by following Godog conventions
- All tests passing with proper response validation
- Maintained black box testing principles

Key files:
- pkg/bdd/steps/steps.go - Step definitions using StepContext struct
- pkg/bdd/testserver/ - Test server implementation
- features/*.feature - BDD feature files
- pkg/bdd/README.md - Documentation for proper step patterns

The implementation follows Godog's exact pattern suggestions to avoid
undefined step warnings and provides comprehensive API testing.

features/greet.feature

@@ -0,0 +1,13 @@
+# features/greet.feature
+Feature: Greet Service
+  The greet service should return appropriate greetings
+
+  Scenario: Default greeting
+    Given the server is running
+    When I request the default greeting
+    Then the response should be "{\"message\":\"Hello world!\"}"
+
+  Scenario: Personalized greeting
+    Given the server is running
+    When I request a greeting for "John"
+    Then the response should be "{\"message\":\"Hello John!\"}"