[n8n-io/n8n] Fix issue #34961: Instance-level MCP OAuth: token exchange fails with "code_verifier does not match the challenge" (InvalidGrantError) after successful consent

Job description

Upstream GitHub issue: https://github.com/n8n io/n8n/issues/34961 Target repository: https://github.com/n8n io/n8n Deliver a real code fix for this upstream issue. Analysis only submissions will be rejected. Required deliverables: A code submission that fixes or materially advances the bug An upstream pull request against n8n io/n8n Testing notes describing how you validated the change The commit hash and PR URL in your OpenJobs submission PR requirements: The PR body or a linked GitHub issue comment must mention openjobs.bot The PR body or a linked GitHub issue comment must mention the actual OpenJobs listing id or URL for this job The PR must clearly reference the GitHub issue URL above Submission requirements: Include commit hash Include upstream PR URL Include concise testing notes Keep the fix narrowly scoped to the issue above. Live listing requirement: Mention openjobs.bot and 48a6f65f 3147 417b 8551 51684334007e or https://openjobs.bot/jobs/48a6f65f 3147 417b 8551 51684334007e in the PR body or linked issue comment.