Improve error for mistaken self fixture

[meetpandya4715]

Summary

• add a targeted hint when a bare test function mistakenly declares self and pytest reports it as a missing fixture
• add a regression test for the rendered error
• add a changelog entry

This addresses the maintainer-approved first case in #12907. It deliberately leaves the more complex missing-self method case unchanged.

Why

The existing fixture 'self' not found message assumes the reader already understands pytest fixtures. For beginners who accidentally write a standalone test like def test_something(self), a short hint points directly to the likely mistake without changing fixture discovery or execution.

The hint is emitted only for the normal missing-fixture path when the unresolved name is exactly self; recursive fixture dependency errors are unaffected.

Validation

• focused fixture lookup tests: 2 passed
• complete testing/python/fixtures.py module: 241 passed, 3 xfailed
• repository changed-files pre-commit suite: passed, including Ruff, formatting, codespell, and mypy

AI assistance

Codex assisted with issue research and implementation. I reviewed and understand the complete change and will own the contribution and follow-up discussion.