fix(init): include requestId in init error response even when empty

Use *string for the RequestId field in ErrorResponse so that an empty
string is serialized (not omitted by omitempty), while nil — used for
fault events — stays omitted. Fixes test_lambda_runtime_error snapshot
mismatch where requestId: "" was expected but absent.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: joe4dev

cmd/localstack/custom_interop.go

@@ -93,7 +93,9 @@ type InvokeRequest struct {
 type ErrorResponse struct {
 	ErrorMessage string   `json:"errorMessage"`
 	ErrorType    string   `json:"errorType,omitempty"`
-	RequestId    string   `json:"requestId,omitempty"`
+	// RequestId uses *string so that an empty string "" is serialized (not omitted),
+	// while nil is omitted — init errors always set this field, fault events leave it nil.
+	RequestId *string  `json:"requestId,omitempty"`
 	StackTrace   []string `json:"stackTrace,omitempty"`
 }
 
@@ -235,10 +237,11 @@ func (c *CustomInteropServer) SendInitErrorResponse(resp *interop.ErrorInvokeRes
 		return c.delegate.SendInitErrorResponse(resp)
 	}
 
+	requestId := c.delegate.GetCurrentInvokeID()
 	adaptedResp := ErrorResponse{
 		ErrorMessage: parsed.ErrorMessage,
 		ErrorType:    parsed.ErrorType,
-		RequestId:    c.delegate.GetCurrentInvokeID(),
+		RequestId:    &requestId,
 		StackTrace:   parsed.StackTrace,
 	}
 	body, err := json.Marshal(adaptedResp)