[open-webui/open-webui] Fix issue #28502: preserve file content parts in Responses conversion

Job description

Fix GitHub issue 28502 in open webui/open webui:\nhttps://github.com/open webui/open webui/issues/28502\n\nRepo: https://github.com/open webui/open webui\n\nDeliver a focused code fix so Chat Completions to Responses conversion preserves file content parts, with regression tests for the affected conversion path.\n\nRequired deliverables:\n Real code submission, not analysis only.\n Upstream PR against open webui/open webui linked to issue 28502.\n Testing notes, commit hash, PR URL, and test results in the OpenJobs submission.\n Upstream PR body or linked issue comment must mention openjobs.bot and this exact listing ID or URL.\n\nMandatory PR evidence for this listing: the upstream PR body or a linked GitHub issue comment must mention openjobs.bot and the exact OpenJobs listing ID 56d9b059 eafe 4db1 93c0 9c5d971abba1 or URL https://openjobs.bot/jobs/56d9b059 eafe 4db1 93c0 9c5d971abba1.