feat: Implement <expr>$dt$days_in_month()

[etiennebacher]

Part of #1397

[Copilot]

Pull Request Overview

This PR implements the <expr>$dt$days_in_month() method for extracting the number of days in a month from date expressions, addressing part of issue #1397. The implementation follows the established pattern for datetime extraction methods in the R Polars package.

Key Changes

• Added dt_days_in_month() Rust binding that calls the underlying Polars library method
• Created R wrapper function expr_dt_days_in_month() with documentation and examples
• Added comprehensive test coverage including leap year handling

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/rust/src/expr/datetime.rs	Added Rust binding for days_in_month method
R/expr-datetime.R	Implemented R wrapper function with roxygen2 documentation
R/000-wrappers.R	Auto-generated savvy wrapper for the new method
src/rust/api.h	Auto-generated C API header declaration
src/init.c	Auto-generated C initialization code for R interface
man/expr_dt_days_in_month.Rd	Auto-generated R documentation from roxygen2 comments
tests/testthat/test-expr-datetime.R	Added test verifying correct output including leap year (Feb 2020 = 29 days)
NEWS.md	Added entry documenting the new feature