fix: resolve #2642 — Query macro documentation understates lack of nullability checking on bind parameters

[chinhkrb113]

Summary

The query! family of macros validates the types of bind parameters against the database's inferred parameter types, but it does not verify whether a bound value's nullability is compatible with the target column's NOT NULL constraint. As a result, passing a None/NULL value for a non-nullable column compiles successfully and only fails at runtime with a database constraint violation. This is a known limitation (databases generally report bind parameters as nullable, and the macro has no way to associate a parameter with a specific column constraint at compile time), but it is not currently documented, which surprised the user. We should add an explicit "Nullability" / "Limitations" note to the macro documentation explaining this behavior so users understand that null-checking is only performed on output columns, not on input bind parameters

Fixes #2642

Changes

• src/macros/mod.rs

Why

src/macros/mod.rs: The query! family of macros validates the types of bind parameters against the database's inferred parameter types, but it does not verify whether a bound value's nullability is compatible with the target column's NOT NULL constraint. As a result, passing a None/NULL value for a non-nullable column compiles successfully and only fails at runtime with a database constraint violation. This is a known limitation (databases generally report bind parameters as nullable, and the macro has no way to associate a parameter with a specific column constraint at compile time), but it is not currently documented, which surprised the user. We should add an explicit "Nullability" / "Limitations" note to the macro documentation explaining this behavior so users understand that null-checking is only performed on output columns, not on input bind parameters.