✨ 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

pkg/bdd/steps/steps.go

@@ -29,6 +29,8 @@ func InitializeAllSteps(ctx *godog.ScenarioContext, client *testserver.Client) {
 	ctx.Step(`^the server is running$`, sc.theServerIsRunning)
 	ctx.Step(`^the server is running with v2 enabled$`, sc.theServerIsRunningWithV2Enabled)
 	ctx.Step(`^I send a POST request to v2 greet with name "([^"]*)"$`, sc.iSendPOSTRequestToV2GreetWithName)
+	ctx.Step(`^I send a POST request to v2 greet with invalid JSON "([^"]*)"$`, sc.iSendPOSTRequestToV2GreetWithInvalidJSON)
+	ctx.Step(`^the response should contain error "([^"]*)"$`, sc.theResponseShouldContainError)
 }
 
 func (sc *StepContext) iRequestAGreetingFor(name string) error {
@@ -91,3 +93,17 @@ func (sc *StepContext) iSendPOSTRequestToV2GreetWithName(name string) error {
 	requestBody := map[string]string{"name": name}
 	return sc.client.Request("POST", "/api/v2/greet", requestBody)
 }
+
+func (sc *StepContext) iSendPOSTRequestToV2GreetWithInvalidJSON(invalidJSON string) error {
+	// Send raw invalid JSON
+	return sc.client.Request("POST", "/api/v2/greet", invalidJSON)
+}
+
+func (sc *StepContext) theResponseShouldContainError(expectedError string) error {
+	// Check if the response contains the expected error
+	body := string(sc.client.GetLastBody())
+	if !strings.Contains(body, expectedError) {
+		return fmt.Errorf("expected response to contain error %q, got %q", expectedError, body)
+	}
+	return nil
+}