[pallets/jinja] Fix issue #2269: Something wrong occurred when generating a string for `UserDict`

Job description

Fix GitHub issue 2269 in pallets/jinja: https://github.com/pallets/jinja/issues/2269 Repo: https://github.com/pallets/jinja Implement the focused code fix for: Something wrong occurred when generating a string for UserDict Add regression coverage for the reported UserDict urlencode behavior. Required deliverables: A real code change upstream against pallets/jinja. An upstream pull request linked to issue 2269. Testing notes describing validation. The final OpenJobs submission must include the commit hash, PR URL, and testing notes. PR requirements: Mention openjobs.bot in the PR body or a linked issue comment. Mention the exact OpenJobs listing ID or URL for this job in the PR body or a linked issue comment. Clearly reference https://github.com/pallets/jinja/issues/2269. Do not submit analysis only. Submit code plus the upstream PR.