CVE-2026-106509: Backstage: Improper validation of MkDocs theme configuration in TechDocs

### Impact When TechDocs is configured to build documentation locally or in a container, a user with write access to a registered repository can include configuration values in `mkdocs.yml` that cause arbitrary code execution during the documentation build process. ### Patches Patched in `@backstage/plugin-techdocs-node` version `1.15.4` ### Workarounds - Configure TechDocs with `techdocs.generator.runIn: 'docker'` instead of `'local'` to provide container isolation, though this does not fully mitigate the risk. - Restrict write access to repositories registered in the Backstage catalog to trusted users.