fix(init): append newline to .gitignore if missing

[ravindra-RKB]

Description

Fixes an issue where pixi init appends .gitignore entries to the last line without a newline, which can break the user's existing last ignore rule (Fixes #7120).
Added a check to ensure that a newline is prepended if the existing .gitignore doesn't end with one.

Checks

• I have checked that this pull request has not already been reported.
• I have added tests to verify that the bug is fixed.
• I have formatted my code using cargo fmt and pixi run lint.

Reproducible example

cd "$(mktemp -d anyname.XXXXX)"
printf 'data/' > .gitignore
pixi init
cat .gitignore

Before:

data/# pixi environments
.pixi/*
!.pixi/config.toml

After:

data/

# pixi environments
.pixi/*
!.pixi/config.toml

This PR doesn't contain AI-generated content.