tests: add conda marker

[henryiii]

Adding a conda marker.

[Copilot]

Pull Request Overview

This PR consolidates conda test configuration by centralizing conda marker handling in the test infrastructure. Instead of using scattered conditional skip decorators, it implements a unified pytest marker system that automatically manages conda test execution.

• Removed local HAS_CONDA variables and has_conda skip decorators from individual test files
• Added centralized conda marker configuration in conftest.py with automatic test discovery and skipping
• Replaced @has_conda decorators with @pytest.mark.conda markers across test files

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
tests/test_virtualenv.py	Removed local conda detection and replaced @has_conda decorators with @pytest.mark.conda markers
tests/test_sessions.py	Removed local conda detection and replaced @has_conda decorator reference with pytest.mark.conda marker
tests/conftest.py	Added centralized conda marker infrastructure with automatic test discovery and conda command blocking
pyproject.toml	Added conda marker definition to pytest configuration

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[theacodes]

Looks fine, but what does the conda marker accomplish?

[henryiii]

This is an easier way to identify and potentially act on the conda tests. Most importantly, it means you can now skip the conda tests even if you have conda installed (pytest -m "not conda"). I believe (it's been a while!) that it came up because I was working on #965, and wanted a way to run only the conda tests (which can't be run in parallel, due to conda) (pytest -m conda), and then all the non-conda tests.

We can also now ensure that tests that call conda are marked (and therefore skipped), even if conda is installed on the developers machine.