Extend test and CI coverage for Laravel 13

[ibourgeois]

Context

As a package maintainer, I need automated confidence that Beacon works on Laravel 13 in local tests and GitHub Actions.

Acceptance Criteria

• the package test matrix includes Laravel 13 in the supported-version coverage plan
• the PHP matrix is updated to reflect practical Laravel 13 support, including the correct minimum PHP version
• forward-compatibility coverage and any nightly assumptions are documented clearly in the workflow or docs

Notes

Keep this issue focused on verification coverage, not package feature work.