fix: preserve New Contributors section with --group-by-type

[Copilot]

GitHub's auto-generated "New Contributors" section was dropped when grouping release notes by conventional commit type.

Root Cause

The serialize() method filtered out lines without a change object. New contributor lines have is_new_contributor=True but change=None, so they were discarded during grouping.

Changes

• models/release_notes.py: Collect new contributor lines in separate list, emit as dedicated section after type-grouped changes
• tests/test_release_notes.py: Add tests for grouped and non-grouped cases

Before/After

Before:

## Features
* Added new dashboard (...)

## Bug Fixes
* Fixed login bug (...)

**Authors**: @alice, @bob

After:

## Features
* Added new dashboard (...)

## Bug Fixes
* Fixed login bug (...)

## New Contributors
* @alice made their first contribution in #1
* @bob made their first contribution in #2

**Authors**: @alice, @bob

The section now appears after all change groups but before Authors/Reviewers, matching GitHub's convention.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• https://api.github.com/repos/nonexistent-owner/nonexistent-repo
  • Triggering command: /usr/bin/python python -m pytest -x --tb=line -v (http block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>The new contributors section gets lost when --group-by-type is used</issue_title>
<issue_description>The section looks roughly like this

## New Contributors
* @barredterra made their first contribution in https://github.com/barredterra/pretty_release_notes/pull/1

We need to preserve it when --group-by-type is active.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes The new contributors section gets lost when --group-by-type is used #1

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