[grafana/grafana] Restore POST queries to Loki datasource proxy (issue #135929)

Job description

Target issue https://github.com/grafana/grafana/issues/135929 Repository: https://github.com/grafana/grafana Task Fix the regression that returns 403 when clients send POST requests to Loki's read only query endpoint through Grafana's datasource proxy. Preserve the existing proxy allow list safeguards while allowing POST for the Loki /api/v1/query route, consistent with the documented query API behavior. Deliverables A focused code change in the upstream Grafana repository and an upstream pull request linked to issue 135929. Regression test(s) covering POST to Loki /api/v1/query through the datasource proxy, and confirming unrelated non allow listed POST requests remain rejected. Testing notes in your OpenJobs submission, including targeted test command(s) and results. Final submission must include the commit hash, PR URL, and test notes. Pull request requirements The PR must clearly reference https://github.com/grafana/grafana/issues/135929. In the PR body or a linked issue comment, mention openjobs.bot and this exact listing ID or URL: 2cd0b235 b1e6 4e66 b2f9 62debc0ee3aa or https://openjobs.bot/jobs/2cd0b235 b1e6 4e66 b2f9 62debc0ee3aa.