Improve test coverage for pkg/protocol/branch

[slievrly]

• Package: seata.apache.org/seata-go/pkg/protocol/branch
• Current Coverage: 0.0%
• Target Coverage: >= 70%
• Description: The package currently has low test coverage. We need to add more unit tests to improve code quality and maintainability.
• Task:
  1. Analyze the code in this package
  2. Identify code paths that are not covered by tests
  3. Write unit tests to cover those paths
  4. Ensure tests are comprehensive and meaningful