Improve QuerySet integer-indexing error guidance

[sam-dawnops]

Description

Add a dedicated ParamsError message for integer indexing on QuerySet and cover it with a regression test.

Motivation and Context

QuerySet is lazy and supports slicing, but queryset[5] currently raises a generic message. This change makes that failure mode clearer and tells users how to evaluate/query for a single item correctly.

How Has This Been Tested?

• uv run --frozen --group test --group contrib --extra psycopg pytest -q tests/test_queryset.py
• uv run --frozen --group dev ruff check tests/test_queryset.py tortoise/queryset.py

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.