feat: Add testTxExec function

This commit adds the `testTxExec` function to the internal package. The function sets up a Redis executable, creates an instrumented RESP connection, and runs a test case for the `EXEC` command.

internal/test_txn_exec.go

@@ -0,0 +1,34 @@
+package internal
+
+import (
+	"github.com/codecrafters-io/redis-tester/internal/redis_executable"
+	"github.com/codecrafters-io/redis-tester/internal/resp_assertions"
+
+	"github.com/codecrafters-io/redis-tester/internal/instrumented_resp_connection"
+	"github.com/codecrafters-io/redis-tester/internal/test_cases"
+	"github.com/codecrafters-io/tester-utils/test_case_harness"
+)
+
+func testTxExec(stageHarness *test_case_harness.TestCaseHarness) error {
+	b := redis_executable.NewRedisExecutable(stageHarness)
+	if err := b.Run(); err != nil {
+		return err
+	}
+
+	logger := stageHarness.Logger
+
+	client, err := instrumented_resp_connection.NewFromAddr(stageHarness, "localhost:6379", "client")
+	if err != nil {
+		logFriendlyError(logger, err)
+		return err
+	}
+	defer client.Close()
+
+	bareExecCommandTestCase := test_cases.SendCommandTestCase{
+		Command:   "EXEC",
+		Args:      []string{},
+		Assertion: resp_assertions.NewErrorAssertion("ERR EXEC without MULTI"),
+	}
+
+	return bareExecCommandTestCase.Run(client, logger)
+}