feat(pytest-plugin): dep extra to track minimum pytest version

[bepri]

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Adds a dependency group to help downstream users with pinning to an appropriate pytest version.

This wasn't a dependency at all in the past, so downstream users just had to coincidentally have pytest already in their env in order to use the plugin.

[Copilot]

🟡 Changes recommended

Narrow the import exception handling and add coverage for the missing-pytest fallback.

2 open findings

• Avoid masking transitive pytest import errors · New
• Add test coverage for missing pytest fallback · New

What changed in this PR

Adds an optional craft-cli[pytest] dependency extra and improves missing-pytest diagnostics.

Changes:

• Declares pytest>=8.4 as an optional dependency.
• Updates the lockfile and changelog.
• Adds import-time error handling for missing pytest.

File	Summary
uv.lock	Records the new optional dependency.
pyproject.toml	Defines the pytest extra.
docs/​changelog.rst	Documents the dependency and error behavior.
craft_cli/​pytest_plugin.py	Adds pytest import handling; the handler should be narrowed and covered by an isolated test.

🧠 Review effort: Lite

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