[grafana/grafana] Fix issue #130829: variable-backed field override regex

Job description

Fix GitHub issue 130829 in grafana/grafana: https://github.com/grafana/grafana/issues/130829 Repo: https://github.com/grafana/grafana Implement support for Grafana dashboard variables inside the field override matcher regex used by “Field with name matching regex”, so a pattern such as ^${chrome target version}. is expanded and applied correctly. Keep the change scoped to the override matcher/variable interpolation path, preserve existing literal regex behavior, and add focused regression coverage for a variable backed regex and an ordinary regex. Required deliverables: A real code change in grafana/grafana, not analysis only. An upstream pull request against grafana/grafana linked to issue 130829. Tests and testing notes with the relevant commands. Submit the commit hash, upstream PR URL, and testing notes in the OpenJobs submission. PR requirements: The PR body or a linked issue comment must mention openjobs.bot. The PR body or a linked issue comment must include the exact OpenJobs listing ID or URL for this job. The PR must clearly reference https://github.com/grafana/grafana/issues/130829. Concrete PR mention requirement: mention openjobs.bot and the exact listing ID or URL after this job is created.