Refactor compression flow with routing seam

Introduce compress_image_with_routing and compress_raster_image to prepare extensible backend routing while keeping existing behavior unchanged.

Add unit test that verifies routing currently delegates to raster compressor.

Co-Authored-By: Abacus.AI CLI <agent@abacus.ai>

test_pptx_image_compress.py

@@ -244,5 +244,34 @@ class TestPptxImageCompress(unittest.TestCase):
             self.assertIn("png_jpg", log_text)
 
 
+    def test_compress_image_with_routing_delegates_to_raster(self):
+        with tempfile.TemporaryDirectory() as td:
+            root = Path(td)
+            original = root / "image1.png"
+            original.write_bytes(b"A" * 100)
+            out_dir = root / "out"
+
+            def fake_compressor(original_path: Path, out_subdir: Path, caesium_threads: int | None, quality: int, min_savings: str):
+                out_subdir.mkdir(parents=True, exist_ok=True)
+                out = out_subdir / original_path.name
+                out.write_bytes(b"B" * 80)
+                return out
+
+            out = pic.compress_image_with_routing(
+                compressor=fake_compressor,
+                original=original,
+                out_dir=out_dir,
+                caesium_threads=1,
+                quality=90,
+                min_savings="2%",
+            )
+
+            self.assertIsNotNone(out)
+            if out is None:
+                self.fail("Output should not be None")
+            self.assertEqual(out.name, "image1.png")
+            self.assertEqual(out.stat().st_size, 80)
+
+
 if __name__ == "__main__":
     unittest.main()