Improve QuerySet integer indexing error guidance

[samdolan]

Description

This PR improves the error guidance for invalid integer indexing on the lazy QuerySet.

• Adds a dedicated ParamsError branch in QuerySet.__getitem__ for integer keys.
• Updates the error message to explain lazy evaluation and actionable alternatives:
  await queryset.first(), await queryset.offset(n).first(), or await queryset.all() and index the returned list.
• Adds test_integer_indexing_not_supported in tests/test_queryset.py to cover this behavior.

No functional change to valid slicing (queryset[start:stop]).

Motivation and Context

QuerySet supports slicing but not list-style integer indexing.
Before this change, users got a generic error that didn’t explain the async/lazy usage pattern clearly.
This update makes the failure mode easier to understand and recover from.

How Has This Been Tested?

• uv run --frozen --group test --group contrib --extra psycopg pytest -q tests/test_queryset.py
  • Result: 64 passed, 8 skipped
• uv run --frozen --group dev ruff check tests/test_queryset.py tortoise/queryset.py
  • Result: All checks passed

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.

[samdolan]

This came up in an interview and the output confused us both a bit. Thought I would pay it forward and have a more discrete error message there.