✨ feat: implement input validation for API v2

- Added go-playground/validator dependency

- Created pkg/validation/ package with custom validator wrapper

- Implemented request validation for v2 greet endpoint

- Added structured validation error responses

- Extended BDD tests to cover validation scenarios

- Updated AGENTS.md with v2 API documentation

- Created ADR 0011-validation-library-selection.md

- Simplified server handler creation code

- Updated CHANGELOG with implementation details

AGENTS.md

@@ -388,6 +388,50 @@ curl http://localhost:8080/api/v1/greet/Alice
 # Response: {"message":"Hello Alice!"}
 ```
 
+### Greet Service v2 (Feature Flag Enabled)
+```http
+POST /api/v2/greet
+```
+
+**Request Body:**
+```json
+{
+  "name": "John"
+}
+```
+
+**Examples:**
+```bash
+# Valid request
+curl -X POST http://localhost:8080/api/v2/greet \
+  -H "Content-Type: application/json" \
+  -d '{"name":"John"}'
+# Response: {"message":"Hello my friend John!"}
+
+# Empty name (valid, returns default)
+curl -X POST http://localhost:8080/api/v2/greet \
+  -H "Content-Type: application/json" \
+  -d '{"name":""}'
+# Response: {"message":"Hello my friend!"}
+
+# Missing name field (valid, returns default)
+curl -X POST http://localhost:8080/api/v2/greet \
+  -H "Content-Type: application/json" \
+  -d '{}'
+# Response: {"message":"Hello my friend!"}
+
+# Name too long (validation error)
+curl -X POST http://localhost:8080/api/v2/greet \
+  -H "Content-Type: application/json" \
+  -d '{"name":"ThisNameIsWayTooLongAndShouldFailValidationBecauseItExceedsTheMaximumAllowedLengthOf100Characters!!!!"}'
+# Response: {"error":"validation_failed","message":"Invalid request data","details":[{"message":"Name failed validation for 'max' (parameter: 100)"}]}'
+```
+
+**Validation Rules:**
+- `name`: Maximum length 100 characters (optional field)
+
+**Feature Flag:** Enable with `DLC_API_V2_ENABLED=true` or in config file with `api.v2_enabled: true`
+
 ## 🔗 OpenTelemetry & Jaeger Integration
 
 The application supports OpenTelemetry for distributed tracing with Jaeger compatibility.