* fix(uploads): delete the requested upload, not a symlink's target
delete_file_safe resolved the requested path before unlinking it. The
uploads directory is writable from local and AIO sandboxes, so a
symlink planted under an upload name was followed: deleting alias.pdf
removed the victim.pdf it pointed to, and the companion cleanup then
removed victim.md, while the link itself survived and the call reported
"Deleted alias.pdf". A link resolving outside the directory was already
refused by the traversal check, so the damage stayed inside the
thread's uploads.
The function now checks and unlinks the requested entry itself and
treats a symlink as not found, the same way list_files_in_dir already
hides it. unlink() never follows the final component, so a file swapped
for a link between the check and the unlink removes only the link.
Tests cover the helper, the Gateway DELETE route, and
DeerFlowClient.delete_upload.
* docs(changelog): note upload delete symlink fix (#5547)
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