Pydantic Fixes and Tests (#311)

* Added SubscriptableBaseModel to the Model classes and added Image codec test

---------

Co-authored-by: Parth Sareen <parth@Parths-MacBook-Pro.local>

tests/test_type_serialization.py

@@ -0,0 +1,15 @@
+from base64 import b64decode, b64encode
+
+from ollama._types import Image
+
+
+def test_image_serialization():
+  # Test bytes serialization
+  image_bytes = b'test image bytes'
+  img = Image(value=image_bytes)
+  assert img.model_dump() == b64encode(image_bytes).decode()
+
+  # Test base64 string serialization
+  b64_str = 'dGVzdCBiYXNlNjQgc3RyaW5n'
+  img = Image(value=b64_str)
+  assert img.model_dump() == b64decode(b64_str).decode()