feat: Add ffmetadata rendering and writing functions with tests for chapter inclusion

tests/test_ffmetadata.py

@@ -0,0 +1,40 @@
+from __future__ import annotations
+
+from pathlib import Path
+
+from abogen.web.conversion_runner import _render_ffmetadata, _write_ffmetadata_file
+
+
+def test_render_ffmetadata_includes_chapters(tmp_path):
+    metadata = {
+        "title": "Sample Book",
+        "artist": "Author Name",
+        "comment": "Line one\nLine two",
+        "publisher": "ACME=Corp",
+    }
+    chapters = [
+        {"start": 0.0, "end": 5.0, "title": "Intro", "voice": "voice_a"},
+        {"start": 5.0, "end": 12.345, "title": "Chapter 2"},
+    ]
+
+    rendered = _render_ffmetadata(metadata, chapters)
+
+    assert ";FFMETADATA1" in rendered
+    assert "title=Sample Book" in rendered
+    assert "artist=Author Name" in rendered
+    assert "comment=Line one\\nLine two" in rendered
+    assert "publisher=ACME\\=Corp" in rendered
+    assert rendered.count("[CHAPTER]") == 2
+    assert "START=0" in rendered
+    assert "END=5000" in rendered
+    assert "voice=voice_a" in rendered
+
+    audio_path = tmp_path / "book.m4b"
+    metadata_path = _write_ffmetadata_file(audio_path, metadata, chapters)
+    assert metadata_path is not None
+    assert metadata_path.exists()
+
+    content = metadata_path.read_text(encoding="utf-8")
+    assert "END=12345" in content
+
+    metadata_path.unlink()