fix: quote literal values for strings in SQLGlot compiler

[sycai]

No description provided.

[gemini-code-assist]

Code Review

This pull request adds support for converting values to string literals when the data type is dtypes.STRING_DTYPE in the SQL compilation module, along with corresponding unit tests. The review feedback points out that directly calling str(value) on PyArrow scalars or null-like objects (such as pd.NA or None) can lead to incorrect SQL generation (e.g., double quotes or string literals like 'None'). It is recommended to unwrap PyArrow scalars using .as_py() and handle nulls properly, as well as add test cases for these scenarios.

[gemini-code-assist]

Directly calling str(value) on any input when dtype == dtypes.STRING_DTYPE can lead to incorrect SQL generation and bugs:

1. If value is a PyArrow scalar (e.g., pa.scalar("hello")), str(value) returns '"hello"' (with literal double quotes), which results in double quotes being embedded in the SQL string literal.
2. If value is a null-like object (such as pa.scalar(None), pd.NA, or None), str(value) will produce string literals like 'None' or '<NA>' instead of SQL NULL.

To prevent this, we should first unwrap PyArrow scalars using .as_py() if available, and then handle null/NA values appropriately before converting to string.

    elif dtype == dtypes.STRING_DTYPE:
        if hasattr(value, "as_py"):
            value = value.as_py()
        if value is None or value is pd.NA:
            return sge.convert(None)
        return sge.convert(str(value))

[gemini-code-assist]

It would be beneficial to add test cases for null/NA values (such as pd.NA and pa.scalar(None)) with STRING_DTYPE to ensure they are correctly compiled to NULL instead of string literals like '<NA>' or 'None'.