Add tests for admin.keys.retrieve() and keys.update()

[MitulShah1]

_KeysResource.retrieve() and .update() (ferrolabsai/admin/resource.py) have no tests. TestAdminKeys currently covers create / revoke / delete / rotate / list / usage only.

What to do

1. Add test_retrieve_key and test_update_key to TestAdminKeys in tests/test_sdk.py.
2. Mock GET /admin/keys/{id} → returns an APIKey dict.
3. Mock PUT /admin/keys/{id} with body params; verify the round-trip.

Pattern to follow

TestAdminKeys.test_list_keys.