[open-webui/open-webui] Fix issue #26904: preserve underline formatting in Channels rich-text send

Job description

Fix GitHub issue 26904 in open webui/open webui: https://github.com/open webui/open webui/issues/26904 Repo: https://github.com/open webui/open webui Target: Fix the rich text send pipeline so underline formatting is preserved consistently, or otherwise implement the upstream approved serialization path instead of silently dropping underline on send. Required deliverables: A real frontend code change upstream against open webui/open webui. An upstream pull request linked to issue 26904. Testing notes covering the affected rich text/Channels send flow. 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. Concrete PR mention requirement for this listing: mention openjobs.bot and 6a987136 b079 4da9 b072 04894210fc63 or https://openjobs.bot/jobs/6a987136 b079 4da9 b072 04894210fc63.