Fix inconsistent .pk behaviour with custom primary keys

[abrookins]

Summary

Fixes #570 - When using a custom primary key field (e.g., x: int = Field(primary_key=True)), accessing .pk on an instance now correctly returns the actual value instead of an ExpressionProxy or auto-generated ULID.

Changes

1. Added PrimaryKeyAccessor descriptor class that:

   • Returns the actual primary key value for instance-level access
   • Returns ExpressionProxy for class-level access (query building)
   • Handles setting the primary key value correctly

2. Modified ModelMeta.__new__ to:

   • Detect when there's exactly one custom primary key (not pk)
   • Remove the default pk field from model_fields
   • Set up the PrimaryKeyAccessor descriptor for .pk access

3. Updated validate_primary_key() to safely handle cases where pk was already removed

Example

class Member(HashModel, index=True):
    x: int = Field(primary_key=True)
    name: str

member = Member(x=42, name="Test")
print(member.pk)  # Now correctly prints: 42 (was: ExpressionProxy before fix)
print(member.x)   # Prints: 42

# Class-level access still works for queries
await Member.find(Member.pk == 42).all()

Testing

• Added tests for both HashModel and JsonModel
• All 226 existing tests pass