chore(cmake): add CMake presets

[henryiii]

Description

This adds CMakePresets, which simplify running CMake for pybind11's own test suite, and improves support for IDEs like VSCode.

Two presets are present:

• cmake --workflow default will build and run tests basically the way we like to do it
• cmake --workflow venv will use a .venv directory, creating it with uv if not already present

I've also added autodiscovery of a .venv directory by default if it's present and nothing specific was requested.

This requires CMake 3.25+ to run with the presets.

I need to work on two more things, as followups:

• Ninja needs to work on Windows. That will enable these presets to be used on the main CI jobs
• We need to move the tests into tests instead of build targets. This would integrate much more elegantly.

Suggested changelog entry:

* Add support for running pybind11's tests via presets in CMake 3.25+.

[Copilot]

Pull Request Overview

This PR adds CMakePresets to simplify running pybind11's test suite and improve IDE support. The changes include:

• Adding CMakePresets.json and updating pyproject.toml to ignore it.
• Introducing new CMake logic in CMakeLists.txt to autodetect and create a virtual environment.
• Updating the CI workflow in .github/workflows/ci.yml to use the new presets.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
pyproject.toml	Updated ignore list to exclude CMakePresets.json
CMakePresets.json	New file defining configuration, build, and workflow presets
CMakeLists.txt	Added logic for autoconfiguring a virtual environment and installing dependencies
.github/workflows/ci.yml	CI workflow updated to leverage the new CMake presets

[cryos]

Looks great.