[huggingface/transformers] Fix issue #47439: Added-token encoder cache goes stale when tokenizers mutate _added_tokens_decoder directly, giving ids that do not round-trip

Job description

Fix GitHub issue 47439 in huggingface/transformers: https://github.com/huggingface/transformers/issues/47439 Repo: https://github.com/huggingface/transformers Target: Ship a real upstream code or documentation fix for this issue in huggingface/transformers. Required deliverables: A real code or documentation change against huggingface/transformers. An upstream pull request linked to issue 47439. Testing notes describing what you validated. 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 the upstream patch plus the PR.