feat: add SupertonicBackend implementing TTSBackend protocol

Encapsulate SupertonicPipeline as an internal detail of
SupertonicBackend. The factory create_supertonic_backend() now
returns a SupertonicBackend instance instead of a raw
SupertonicPipeline, satisfying the TTSBackend protocol with
metadata, synthesize, get_available_voices, get_supported_formats,
and get_info methods. Backward-compatible __call__ delegates to
the internal pipeline.

tests/test_tts_supertonic_unsupported_chars.py

@@ -1,6 +1,6 @@
 import numpy as np
 
-from abogen.tts_backends.supertonic import SupertonicPipeline
+from abogen.tts_backends.supertonic import SupertonicBackend, SupertonicPipeline
 
 
 class _DummyTTS:
@@ -26,13 +26,23 @@ class _DummyTTS:
         return audio, 0.05
 
 
-def test_supertonic_pipeline_strips_unsupported_characters_and_retries():
-    # Avoid importing/initializing real supertonic by manually constructing the pipeline.
+def _make_pipeline() -> SupertonicPipeline:
     pipeline = SupertonicPipeline.__new__(SupertonicPipeline)
     pipeline.sample_rate = 24000
     pipeline.total_steps = 5
     pipeline.max_chunk_length = 1000
     pipeline._tts = _DummyTTS()
+    return pipeline
+
+
+def _make_backend() -> SupertonicBackend:
+    backend = SupertonicBackend.__new__(SupertonicBackend)
+    backend._pipeline = _make_pipeline()
+    return backend
+
+
+def test_supertonic_pipeline_strips_unsupported_characters_and_retries():
+    pipeline = _make_pipeline()
 
     segs = list(pipeline("Hello • world", voice="M1", speed=1.0))
     assert len(segs) == 1
@@ -43,11 +53,56 @@ def test_supertonic_pipeline_strips_unsupported_characters_and_retries():
 
 
 def test_supertonic_pipeline_drops_chunk_if_only_unsupported_characters():
-    pipeline = SupertonicPipeline.__new__(SupertonicPipeline)
-    pipeline.sample_rate = 24000
-    pipeline.total_steps = 5
-    pipeline.max_chunk_length = 1000
-    pipeline._tts = _DummyTTS()
+    pipeline = _make_pipeline()
 
     segs = list(pipeline("•", voice="M1", speed=1.0))
     assert segs == []
+
+
+# --- SupertonicBackend tests ---
+
+
+def test_backend_metadata():
+    backend = _make_backend()
+    meta = backend.metadata
+    assert meta.id == "supertonic"
+    assert meta.name == "SuperTonic"
+    assert "SuperTonic" in meta.description
+
+
+def test_backend_get_available_voices():
+    backend = _make_backend()
+    voices = backend.get_available_voices()
+    assert isinstance(voices, list)
+    assert "M1" in voices
+    assert "F1" in voices
+
+
+def test_backend_get_supported_formats():
+    backend = _make_backend()
+    formats = backend.get_supported_formats()
+    assert "wav" in formats
+
+
+def test_backend_get_info():
+    backend = _make_backend()
+    info = backend.get_info()
+    assert info["sample_rate"] == 24000
+    assert info["total_steps"] == 5
+    assert isinstance(info["voices"], list)
+
+
+def test_backend_call_delegates_to_pipeline():
+    backend = _make_backend()
+    segs = list(backend("Hello • world", voice="M1", speed=1.0))
+    assert len(segs) == 1
+    assert segs[0].audio.size > 0
+
+
+def test_backend_synthesize_returns_wav_bytes():
+    backend = _make_backend()
+    wav_bytes = backend.synthesize("Hello world", voice="M1", speed=1.0)
+    assert isinstance(wav_bytes, bytes)
+    assert len(wav_bytes) > 0
+    # WAV magic number
+    assert wav_bytes[:4] == b"RIFF"