refactor: migrate FFmpeg metadata functions to infrastructure/exporters.py

- Extract FFmpeg metadata functions to infrastructure/exporters.py as ExportService
- _escape_ffmetadata_value → _escape_ffmetadata_value
- _render_ffmetadata → render_ffmetadata
- _write_ffmetadata_file → write_ffmetadata_file
- _metadata_to_ffmpeg_args → _metadata_to_ffmpeg_args
- _apply_m4b_chapters_with_mutagen → _apply_m4b_chapters_mutagen
- _embed_m4b_metadata → embed_m4b_metadata
- Add tests/test_exporters.py with 28 tests for ExportService
- Update tests/test_ffmetadata.py to use ExportService
- Update conversion_runner.py to use ExportService
- All tests pass with new implementation matching old behavior

tests/test_exporters.py

@@ -0,0 +1,163 @@
+"""Tests for ExportService FFmpeg metadata methods."""
+import os
+import sys
+sys.path.insert(0, os.path.abspath(os.path.join(os.path.dirname(__file__), "..")))
+
+from pathlib import Path
+from abogen.infrastructure.exporters import ExportService
+
+
+class TestEscapeFfmetadataValue:
+    def setup_method(self):
+        self.svc = ExportService()
+
+    def test_simple_string(self):
+        assert self.svc._escape_ffmetadata_value("hello") == "hello"
+
+    def test_escapes_backslash(self):
+        assert self.svc._escape_ffmetadata_value("a\\b") == "a\\\\b"
+
+    def test_escapes_newline(self):
+        assert self.svc._escape_ffmetadata_value("line1\nline2") == "line1\\nline2"
+
+    def test_escapes_equals(self):
+        assert self.svc._escape_ffmetadata_value("key=value") == "key\\=value"
+
+    def test_escapes_semicolon(self):
+        assert self.svc._escape_ffmetadata_value("a;b") == "a\\;b"
+
+    def test_escapes_hash(self):
+        assert self.svc._escape_ffmetadata_value("#comment") == "\\#comment"
+
+    def test_escapes_all_special(self):
+        result = self.svc._escape_ffmetadata_value("a\\b\nc=d;e#f")
+        assert "\\\\" in result
+        assert "\\n" in result
+        assert "\\=" in result
+        assert "\\;" in result
+        assert "\\#" in result
+
+    def test_empty_string(self):
+        assert self.svc._escape_ffmetadata_value("") == ""
+
+
+class TestRenderFfmetadata:
+    def setup_method(self):
+        self.svc = ExportService()
+
+    def test_renders_header(self):
+        result = self.svc.render_ffmetadata({"title": "My Book"}, [])
+        assert result.startswith(";FFMETADATA1\n")
+        assert "title=My Book\n" in result
+
+    def test_renders_multiple_keys(self):
+        result = self.svc.render_ffmetadata({"title": "T", "artist": "A"}, [])
+        assert "title=T\n" in result
+        assert "artist=A\n" in result
+
+    def test_skips_none_values(self):
+        result = self.svc.render_ffmetadata({"title": None}, [])
+        assert "title=" not in result
+
+    def test_renders_chapters(self):
+        chapters = [{"start": 0.0, "end": 10.0, "title": "Ch 1"}]
+        result = self.svc.render_ffmetadata({}, chapters)
+        assert "[CHAPTER]" in result
+        assert "TIMEBASE=1/1000" in result
+        assert "START=0" in result
+        assert "END=10000" in result
+        assert "title=Ch 1" in result
+
+    def test_renders_voice_in_chapter(self):
+        chapters = [{"start": 0.0, "end": 5.0, "voice": "af_heart"}]
+        result = self.svc.render_ffmetadata({}, chapters)
+        assert "voice=af_heart" in result
+
+    def test_skips_chapters_without_times(self):
+        chapters = [{"title": "No times"}]
+        result = self.svc.render_ffmetadata({}, chapters)
+        assert "[CHAPTER]" not in result
+
+    def test_end_equals_start_gets_minimum_duration(self):
+        chapters = [{"start": 5.0, "end": 5.0, "title": "Zero"}]
+        result = self.svc.render_ffmetadata({}, chapters)
+        assert "START=5000" in result
+        assert "END=5001" in result
+
+    def test_empty_metadata_and_chapters(self):
+        result = self.svc.render_ffmetadata({}, [])
+        assert result.strip() == ";FFMETADATA1"
+
+    def test_escapes_special_chars_in_title(self):
+        chapters = [{"start": 0.0, "end": 1.0, "title": "A=B;C#D"}]
+        result = self.svc.render_ffmetadata({}, chapters)
+        assert "\\=" in result
+        assert "\\;" in result
+        assert "\\#" in result
+
+    def test_negative_start_clamped_to_zero(self):
+        chapters = [{"start": -1.0, "end": 5.0, "title": "Neg"}]
+        result = self.svc.render_ffmetadata({}, chapters)
+        assert "START=0" in result
+
+
+class TestMetadataToFfmpegArgs:
+    def setup_method(self):
+        self.svc = ExportService()
+
+    def test_simple_metadata(self):
+        args = self.svc._metadata_to_ffmpeg_args({"title": "My Book"})
+        assert args == ["-metadata", "title=My Book"]
+
+    def test_year_becomes_date(self):
+        args = self.svc._metadata_to_ffmpeg_args({"year": "2024"})
+        assert args == ["-metadata", "date=2024"]
+
+    def test_skips_none_and_empty(self):
+        args = self.svc._metadata_to_ffmpeg_args({"title": None, "artist": ""})
+        assert args == []
+
+    def test_skips_empty_key(self):
+        args = self.svc._metadata_to_ffmpeg_args({"": "value"})
+        assert args == []
+
+    def test_multiple_keys(self):
+        args = self.svc._metadata_to_ffmpeg_args({"title": "T", "artist": "A"})
+        assert "-metadata" in args
+        assert "title=T" in args
+        assert "artist=A" in args
+
+    def test_empty_metadata(self):
+        assert self.svc._metadata_to_ffmpeg_args({}) == []
+
+    def test_none_metadata(self):
+        assert self.svc._metadata_to_ffmpeg_args(None) == []
+
+
+class TestWriteFfmetadataFile:
+    def setup_method(self):
+        self.svc = ExportService()
+
+    def test_writes_file(self, tmp_path):
+        audio = tmp_path / "test.mp3"
+        audio.touch()
+        meta = {"title": "My Book"}
+        chapters = [{"start": 0.0, "end": 5.0, "title": "Ch 1"}]
+        result = self.svc.write_ffmetadata_file(audio, meta, chapters)
+        assert result is not None
+        assert result.exists()
+        content = result.read_text()
+        assert ";FFMETADATA1" in content
+        assert "title=My Book" in content
+
+    def test_returns_none_for_empty(self, tmp_path):
+        audio = tmp_path / "test.mp3"
+        audio.touch()
+        result = self.svc.write_ffmetadata_file(audio, {}, [])
+        assert result is None
+
+    def test_returns_none_for_only_header(self, tmp_path):
+        audio = tmp_path / "test.mp3"
+        audio.touch()
+        result = self.svc.write_ffmetadata_file(audio, None, None)
+        assert result is None