feat: Enhance voice formula parsing and validation, implement voice asset caching, and add tests for new functionality

tests/test_voice_cache.py

@@ -0,0 +1,56 @@
+from types import SimpleNamespace
+from typing import cast
+
+import pytest
+
+from abogen.constants import VOICES_INTERNAL
+from abogen.voice_cache import _CACHED_VOICES, ensure_voice_assets
+from abogen.web.conversion_runner import _collect_required_voice_ids
+from abogen.web.service import Job
+
+
+@pytest.fixture(autouse=True)
+def clear_voice_cache():
+    _CACHED_VOICES.clear()
+    yield
+    _CACHED_VOICES.clear()
+
+
+def test_ensure_voice_assets_downloads_missing(monkeypatch):
+    recorded = []
+
+    def fake_download(**kwargs):
+        recorded.append(kwargs["filename"])
+        return "/tmp/fake"
+
+    monkeypatch.setattr("abogen.voice_cache.hf_hub_download", fake_download)
+
+    downloaded, errors = ensure_voice_assets(["af_nova", "am_liam"])
+
+    assert downloaded == {"af_nova", "am_liam"}
+    assert errors == {}
+    assert recorded == ["voices/af_nova.pt", "voices/am_liam.pt"]
+
+    recorded.clear()
+    downloaded_again, errors_again = ensure_voice_assets(["af_nova"])
+
+    assert downloaded_again == set()
+    assert errors_again == {}
+    assert recorded == []
+
+
+def test_collect_required_voice_ids_includes_all():
+    job = SimpleNamespace(
+        voice="af_nova",
+        chapters=[{"voice_formula": "af_nova*0.7+am_liam*0.3"}],
+        chunks=[{"voice": "am_michael"}],
+        speakers={
+            "hero": {"voice_formula": "af_nova*0.6+am_liam*0.4"},
+            "narrator": {"voice": "af_nova"},
+        },
+    )
+
+    voices = _collect_required_voice_ids(cast(Job, job))
+
+    assert {"af_nova", "am_liam", "am_michael"}.issubset(voices)
+    assert voices.issuperset(VOICES_INTERNAL)