Use the internal pretty-printer consistently - #14904

Closes #14859.

Summary

• replace the remaining stdlib "pformat" call sites with pytest's internal "PrettyPrinter"
• use "pformat_lines" for line-oriented output, including the existing truncation budgets for mapping diffs
• keep mapping summaries deterministically sorted while formatting nested values in pytest's expanded style
• add regression coverage and update assertion-output snapshots for the expanded representation

This makes nested output consistent across supported Python versions instead of changing only on Python 3.15+.

Validation

• 523 focused tests passed
• all pre-commit hooks passed, including Ruff, mypy, codespell, and changelog checks

AI-assisted contribution

This implementation was prepared with OpenAI Codex. I reviewed the behavior and diff, understand the output and truncation changes, and take responsibility for maintaining the contribution and responding substantively to review feedback.