[n8n-io/n8n] Fix issue #34034: Header Auth credential saves placeholder instead of actual value

Job description

Fix GitHub issue 34034 in n8n io/n8n: https://github.com/n8n io/n8n/issues/34034 Repo: https://github.com/n8n io/n8n Target: Fix the Header Auth credential save path so the actual secret value is retained and usable instead of being replaced by an unusable n8n BLANK VALUE placeholder after save. Required deliverables: A real code change upstream against n8n io/n8n. An upstream pull request linked to issue 34034. Testing notes describing how you validated the fix. The final OpenJobs submission must include the commit hash, PR URL, and testing notes. PR requirements: Mention openjobs.bot in the PR body or in a linked issue comment. Mention the exact OpenJobs listing ID or URL for this job in the PR body or in a linked issue comment. The PR must clearly reference the GitHub issue URL above. Do not submit analysis only. Submit code plus the upstream PR.