Prevent EmbeddedJsonModel from generating primary keys

[abrookins]

Summary

Fixes #496 - EmbeddedJsonModel was generating primary keys with ULIDs even though embedded models don't need them (they're stored as part of their parent document, not as separate Redis keys).

Changes

1. EmbeddedJsonModel.pk field: Override to use Field(default=None, exclude=True) so pk is excluded from serialization
2. validate_pk validator: Skip pk generation for embedded models (return None)
3. validate_primary_key: Skip primary key validation for embedded models
4. Metaclass: Clear _meta.primary_key for embedded models
5. Test update: Updated test_schema to reflect that embedded model pk fields are no longer in the RediSearch schema

Before

from aredis_om import EmbeddedJsonModel

class Address(EmbeddedJsonModel):
    city: str

addr = Address(city="Portland")
print(addr.pk)  # 01KFXVST2B7R8ZWBDZN21KX24P (unwanted ULID)
print(addr.model_dump())  # {'pk': '01KFXVST2B7R8ZWBDZN21KX24P', 'city': 'Portland'}

After

from aredis_om import EmbeddedJsonModel

class Address(EmbeddedJsonModel):
    city: str

addr = Address(city="Portland")
print(addr.pk)  # None
print(addr.model_dump())  # {'city': 'Portland'}