Preserve primary key when loading models

[sakshichitnis27]

Fixes #828

Summary

Preserve the requested primary key when loading HashModel and JsonModel records with a missing stored primary key.

Added tests for default and custom primary keys.

Testing

• pytest -q tests/test_primary_key_loading.py tests_sync/test_primary_key_loading.py

---

Note

Low Risk
Small, read-path change in get() with focused tests; behavior only affects records missing a stored PK and should not alter keys that already include one.

Overview
HashModel.get and JsonModel.get now inject the lookup key into the document before Pydantic validation when Redis returns a record without a stored primary key field (common for legacy data).

The primary key field name comes from cls._meta.primary_key.name, so default pk and custom Field(primary_key=True) fields both resolve correctly. The HashModel byte-decode fallback path sets the same field after decoding.

New async tests cover missing stored PK for hash and JSON models, bytes responses, and a custom primary key.

^{Reviewed by Cursor Bugbot for commit 010fa7b. Bugbot is set up for automated code reviews on this repo. Configure here.}

[cursor]

Cursor Bugbot has reviewed your changes using default effort and found 2 potential issues.

^{Reviewed by Cursor Bugbot for commit eea94cf. Configure here.}