Skip installed inline dependencies

[tobim]

🔍 Problem

• Inline dependencies for Python tests and project fixtures are always installed with uv pip install.
• Pre-provisioned environments, such as Nix checks, may already provide those packages through the active Python environment.
• In those environments, attempting a runtime install can fail before fixture loading reaches test selection.
• Some callers also need a hard opt-out when they manage dependency provisioning themselves.

🛠️ Solution

• Treat bare inline dependency names as satisfied when importlib.metadata can find an installed distribution.
• Add TENZIR_TEST_DISABLE_INLINE_DEPENDENCY_INSTALL to skip inline dependency installation entirely.
• Delay the uv lookup until there are actual missing dependencies to install.
• Preserve the existing behavior for versioned requirement strings by default, which still go through uv.
• Add tests for the pre-installed bare dependency path, the explicit opt-out, and the versioned requirement path.
• Document the behavior and add an unreleased changelog entry.

💬 Review

• The installed-package check is intentionally conservative: only bare package names are skipped.
• The environment variable is a caller-controlled escape hatch; fixture imports can still fail later if required packages are not actually present.
• Requirement specifier handling remains delegated to uv unless installation is explicitly disabled.

_{📚 Docs PR: tenzir/docs#361}

[mavam]

@tobim Why did you undo my addition of the changelog entry and the docs fix? Did you force-push accidentally?

[tobim]

@tobim Why did you undo my addition of the changelog entry and the docs fix? Did you force-push accidentally?

My agent did without asking :(.

[mavam]

Then please redo the changes yourself, I deleted my branch already.