docs: clarify subtests progress output

[1Utkarsh1]

Summary

• Render the subtests example output as plain text so the docs do not color every u progress marker as passing.
• Add a short note explaining that the compact u marker is used for both passed and failed subtests, and that the summary shows which subtests failed.
• Add a short changelog fragment for the documentation clarification.

Closes #13902.

Testing

• git diff --check
• uv run --python 3.14 --with tox tox -e docs -- -q

Notes

• The commit includes an AI co-author trailer, per the project PR template.

[Copilot]

Pull request overview

This PR updates pytest’s subtests documentation to avoid misleading syntax highlighting in the rendered example output, and adds a brief explanation of the u progress marker behavior, with a corresponding changelog note (closes #13902).

Changes:

• Switch the subtests example output block to plain text rendering to prevent all u markers from being colored as “passing”.
• Add a note explaining that u is used for both passed and failed subtests and to consult the short summary for failures.
• Add a documentation changelog fragment describing the clarification.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
doc/en/how-to/subtests.rst	Renders the sample output as plain text and explains the u marker semantics to avoid misleading coloration.
changelog/13902.doc.rst	Adds a newsfragment noting the documentation clarification.

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[nicoddemus]

Thanks!

[patchback]

Backport to 9.0.x: 💚 backport PR created

✅ Backport PR branch: patchback/backports/9.0.x/7df5d80ff3a98714a1d3cdbe82941229e511f4b3/pr-14467

Backported as #14468

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