refactor: extract metadata helpers into domain/metadata_helpers.py

- Extract normalize_metadata_map, format_author_sentence, ensure_sentence
- Extract normalize_series_number, extract_series_metadata, format_series_sentence
- Remove _SERIES_NAME_KEYS, _SERIES_NUMBER_KEYS, _SERIES_NUMBER_RE from conversion_runner.py
- Add tests/test_metadata_helpers.py with 32 tests
- All tests match old behavior

tests/test_metadata_helpers.py

@@ -0,0 +1,133 @@
+"""Tests for domain/metadata_helpers.py."""
+import sys
+import os
+sys.path.insert(0, os.path.abspath(os.path.join(os.path.dirname(__file__), "..")))
+
+from abogen.domain.metadata_helpers import (
+    normalize_metadata_map,
+    format_author_sentence,
+    ensure_sentence,
+    normalize_series_number,
+    extract_series_metadata,
+    format_series_sentence,
+)
+
+
+class TestNormalizeMetadataMap:
+    def test_empty(self):
+        assert normalize_metadata_map({}) == {}
+
+    def test_none(self):
+        assert normalize_metadata_map(None) == {}
+
+    def test_normalizes_keys(self):
+        result = normalize_metadata_map({"Title": "My Book", "artist": "John"})
+        assert "title" in result
+        assert "artist" in result
+
+    def test_skips_none_values(self):
+        result = normalize_metadata_map({"title": None, "artist": "John"})
+        assert "title" not in result
+
+    def test_skips_empty_values(self):
+        result = normalize_metadata_map({"title": "", "artist": "John"})
+        assert "title" not in result
+
+
+class TestFormatAuthorSentence:
+    def test_none(self):
+        assert format_author_sentence(None) == ""
+
+    def test_empty(self):
+        assert format_author_sentence("") == ""
+
+    def test_unknown(self):
+        assert format_author_sentence("Unknown") == ""
+
+    def test_single(self):
+        assert format_author_sentence("John Doe") == "By John Doe"
+
+    def test_two(self):
+        assert format_author_sentence("John, Jane") == "By John and Jane"
+
+    def test_three(self):
+        assert format_author_sentence("John, Jane, Bob") == "By John, Jane, and Bob"
+
+    def test_ampersand(self):
+        assert format_author_sentence("John & Jane") == "By John and Jane"
+
+
+class TestEnsureSentence:
+    def test_empty(self):
+        assert ensure_sentence("") == ""
+
+    def test_already_sentence(self):
+        assert ensure_sentence("Hello.") == "Hello."
+
+    def test_adds_period(self):
+        assert ensure_sentence("Hello") == "Hello."
+
+    def test_exclamation(self):
+        assert ensure_sentence("Hello!") == "Hello!"
+
+
+class TestNormalizeSeriesNumber:
+    def test_empty(self):
+        assert normalize_series_number("") is None
+
+    def test_integer(self):
+        assert normalize_series_number("3") == "3"
+
+    def test_float(self):
+        assert normalize_series_number("3.5") == "3.5"
+
+    def test_float_trailing_zero(self):
+        assert normalize_series_number("3.10") == "3.1"
+
+    def test_comma_as_separator(self):
+        assert normalize_series_number("3,5") == "3.5"
+
+    def test_text_with_number(self):
+        assert normalize_series_number("Book 3") == "3"
+
+    def test_none(self):
+        assert normalize_series_number(None) is None
+
+
+class TestExtractSeriesMetadata:
+    def test_empty(self):
+        name, number = extract_series_metadata({})
+        assert name is None
+        assert number is None
+
+    def test_series_name(self):
+        name, number = extract_series_metadata({"series": "My Series"})
+        assert name == "My Series"
+        assert number is None
+
+    def test_series_number(self):
+        name, number = extract_series_metadata({"series_index": "3"})
+        assert name is None
+        assert number == "3"
+
+    def test_both(self):
+        name, number = extract_series_metadata({"series": "My Series", "series_index": "3"})
+        assert name == "My Series"
+        assert number == "3"
+
+
+class TestFormatSeriesSentence:
+    def test_empty(self):
+        assert format_series_sentence(None, None) == ""
+
+    def test_name_only(self):
+        assert format_series_sentence("My Series", None) == ""
+
+    def test_number_only(self):
+        assert format_series_sentence(None, "3") == ""
+
+    def test_both(self):
+        assert format_series_sentence("My Series", "3") == "Book 3 of the My Series"
+
+    def test_with_the(self):
+        assert format_series_sentence("The Lord of the Rings", "1") == "Book 1 of The Lord of the Rings"