feat: Implement voice fallback logic and enhance voice resolution tests for custom mixes

tests/test_conversion_voice_resolution.py

@@ -0,0 +1,52 @@
+from types import SimpleNamespace
+from typing import cast
+
+from abogen.constants import VOICES_INTERNAL
+from abogen.web.conversion_runner import (
+    _chapter_voice_spec,
+    _chunk_voice_spec,
+    _collect_required_voice_ids,
+)
+from abogen.web.service import Job
+
+
+def _sample_job(formula: str) -> Job:
+    return cast(
+        Job,
+        SimpleNamespace(
+        voice="__custom_mix",
+        speakers={
+            "narrator": {
+                "resolved_voice": formula,
+            }
+        },
+        chapters=[],
+        chunks=[{}],
+        ),
+    )
+
+
+def test_chapter_voice_spec_uses_resolved_formula():
+    formula = "af_nova*0.7+am_liam*0.3"
+    job = _sample_job(formula)
+
+    assert _chapter_voice_spec(job, None) == formula
+
+
+def test_chunk_voice_fallback_uses_resolved_formula():
+    formula = "af_nova*0.7+am_liam*0.3"
+    job = _sample_job(formula)
+
+    result = _chunk_voice_spec(job, {}, "")
+
+    assert result == formula
+
+
+def test_voice_collection_includes_formula_components():
+    formula = "af_nova*0.7+am_liam*0.3"
+    job = _sample_job(formula)
+
+    voices = _collect_required_voice_ids(job)
+
+    assert {"af_nova", "am_liam"}.issubset(voices)
+    assert voices.issuperset(VOICES_INTERNAL)