Fix HashModel list validation blocking vector fields

[abrookins]

Summary

Fixes #544 - HashModel incorrectly rejected list[float] fields even when they were vector fields that require this type annotation.

Changes

1. Modified HashModel.__init_subclass__ to check if a field has vector_options before raising the "cannot index list" error. Vector fields are now allowed to use list[float] type.

2. Added vector serialization in save() - Vector fields (list[float]) are automatically serialized to packed float32 bytes before storage, since Redis Hash fields can only store scalar values.

3. Added vector deserialization in get() - Packed bytes are automatically converted back to list[float] when retrieving vector fields.

Example Usage

from aredis_om import HashModel, Field, VectorFieldOptions

vector_options = VectorFieldOptions.flat(
    type=VectorFieldOptions.TYPE.FLOAT32,
    dimension=384,
    distance_metric=VectorFieldOptions.DISTANCE_METRIC.COSINE,
)

class Document(HashModel, index=True):
    name: str
    embedding: list[float] = Field(default=[], vector_options=vector_options)

# Now works correctly:
doc = Document(name="test", embedding=[0.1, 0.2, 0.3, ...])
await doc.save()
retrieved = await Document.get(doc.pk)
# retrieved.embedding is list[float]

Testing

• Added test test_hashmodel_vector_field_with_list to verify the fix
• All existing tests pass (63 hash model tests, 2 KNN tests)