feat: Implement <group_by/lazy_group_by>$len()

[etiennebacher]

Part of #1397

[Copilot]

Pull Request Overview

This PR implements the len() method for both GroupBy and LazyGroupBy classes to return the number of rows in each group, contributing to issue #1397.

Key Changes

• Added <group_by>$len() and <lazy_group_by>$len() methods that return row counts per group
• Included optional name parameter to customize the output column name (defaults to "len")
• Comprehensive test coverage with both valid usage and error cases

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
R/lazyframe-group_by.R	Implements lazygroupby__len() method with proper documentation
R/dataframe-group_by-general.R	Implements groupby__len() method with documentation inheritance issue
man/lazygroupby__len.Rd	Generated documentation for lazy group_by len method
man/groupby__len.Rd	Generated documentation for group_by len method (contains incorrect title due to wrong inheritance)
tests/testthat/test-lazyframe-frame.R	Adds comprehensive tests for both DataFrame and LazyFrame via expect_query_equal
tests/testthat/_snaps/lazyframe-frame.md	Captures expected error messages for invalid parameter types
altdoc/mkdocs.yml	Updates navigation to include new len methods in documentation
NEWS.md	Documents the new feature in changelog

[eitsupi]

Thank you.

I know this is a little late, but the methods for groupby and lazygroupby are exactly the same, so doing something like groupby__len <- lazygroupby__len seems sufficient.

[etiennebacher]

Ah yes indeed, it seems this is the case for many (all?) group_by functions. Is it fine if I do this change everywhere possible in a followup PR?

[eitsupi]

That's good, thanks. I'll merge it for now.