Refactor: Add explicit type annotation for input buffer in Windows input handling

[pyob-bot]

Summary of Changes

• Updated _win32_input method in src/pyob/core_utils.py to include an explicit type annotation for the input_str variable (input_str: str = "").
• This change clarifies the intended type of the buffer used to accumulate user input on Windows platforms.

Technical Impact

• Improves code readability and static analysis support by making the variable's type explicit.
• Enhances compatibility with type checkers such as mypy and IDEs, reducing potential type‑related warnings.
• No functional behavior changes; the method operates identically, merely with clearer type intent.