feat: Add ErrorAssertion for comparing error values in tests

internal/resp/value/value.go

@@ -96,6 +96,10 @@ func (v *Value) Integer() int {
 	return v.integer
 }
 
+func (v *Value) Error() string {
+	return string(v.String())
+}
+
 func (v *Value) FormattedString() string {
 	switch v.Type {
 	case SIMPLE_STRING:

internal/resp_assertions/error_assertion.go

@@ -0,0 +1,27 @@
+package resp_assertions
+
+import (
+	"fmt"
+
+	resp_value "github.com/codecrafters-io/redis-tester/internal/resp/value"
+)
+
+type ErrorAssertion struct {
+	ExpectedValue string
+}
+
+func NewErrorAssertion(expectedValue string) RESPAssertion {
+	return ErrorAssertion{ExpectedValue: expectedValue}
+}
+
+func (a ErrorAssertion) Run(value resp_value.Value) error {
+	if value.Type != resp_value.ERROR {
+		return fmt.Errorf("Expected error, got %s", value.Type)
+	}
+
+	if value.Error() != a.ExpectedValue {
+		return fmt.Errorf("Expected %s, got %s", a.ExpectedValue, value.Error())
+	}
+
+	return nil
+}