feat: implement IS DISTINCT FROM expression

[tisonkun]

No description provided.

[Copilot]

Pull request overview

This pull request implements the SQL IS DISTINCT FROM and IS NOT DISTINCT FROM operators for the PostgreSQL query builder. These operators are useful for null-safe comparisons where NULL values are treated as comparable values (unlike regular equality operators where NULL = NULL is false).

Changes:

• Added is_distinct_from() and is_not_distinct_from() methods to the Expr API
• Added IsDistinctFrom and IsNotDistinctFrom enum variants to BinaryOp
• Updated SQL rendering and operator precedence logic to support the new operators

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
pqb/src/expr.rs	Added implementation of IS DISTINCT FROM operators including enum variants, public API methods, SQL rendering, and precedence handling
pqb/tests/expr.rs	Added comprehensive test coverage for the new operators with basic usage and complex expression scenarios

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