Plugins that add CLI options via tox_add_option couldn't be used with requires-based provisioning. Running tox --demo-plugin where --demo-plugin is defined by a plugin in requires would fail with "unrecognized arguments" because CLI argument validation happened before provisioning had a chance to install the plugin and re-execute tox.

🔧 The fix defers unknown argument validation until after the provisioning check. During initial CLI parsing, parse_known_args is used instead of parse_args, allowing unrecognized flags through. If provisioning is needed, tox re-executes in the provisioned environment where the plugin is installed and the flag is recognized. If provisioning is not needed, any remaining unknown arguments still raise the familiar error message — so typos and invalid flags are still caught.

The demo plugin used in integration tests now also registers a --demo-plugin CLI flag via tox_add_option to exercise this path.

Fixes #2935