test: add unit tests for UserService

[zhangshun-123]

No description provided.

[peter-jerry-ye-code-review]

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The pull request appears to add test cases for the BytesReader implementation, specifically testing the read_char and peek_char functions. The tests cover various scenarios, including handling ASCII characters, surrogate pairs, and incomplete surrogate pairs.

Here are the review points:

💡 [Add documentation for test cases]

• Category: Maintainability
• Code Snippet: All test functions
• Recommendation: Add a brief comment above each test function explaining what specific scenario it is testing.
• Reasoning: While the test names are descriptive, adding comments can help future maintainers quickly understand the purpose of each test, especially for edge cases like surrogate pairs.

⚠️ [Potential bug in incomplete surrogate handling]

• Category: Correctness
• Code Snippet: inspect!(reader.read_char(), content="Some('')") and inspect!(reader.peek_char(), content="Some('')")
• Recommendation: Verify that the replacement character '' is the expected behavior for incomplete surrogate pairs. If possible, handle the error more explicitly (e.g., return an error type).
• Reasoning: The current implementation seems to replace incomplete surrogate pairs with a replacement character, but this behavior should be explicitly documented and tested. It would be better to handle such cases in a way that is consistent with the overall design of the codebase.

💡 [Consider adding more test cases]

• Category: Maintainability
• Code Snippet: All test functions
• Recommendation: Consider adding more test cases, such as handling multi-byte characters, empty strings, and strings with mixed ASCII and non-ASCII characters.
• Reasoning: The current test suite is good, but additional scenarios could ensure more comprehensive coverage. For example, testing a string with both ASCII and surrogate pair characters would be valuable.

Overall, the changes are well-thought-out and add valuable test coverage to the BytesReader implementation. The main improvement would be to add more documentation and ensure the handling of incomplete surrogate pairs is well-defined.