The video edit endpoint parsed the multipart body but dropped the uploaded
source video, only normalizing it to an id. When a raw file is uploaded it now
flows through videos.main -> the http handler -> the provider transform, which
emits multipart/form-data with the source video as a file part, matching the
official OpenAI SDK's videos.edit wire format. Edit-by-id still egresses JSON.
``variant`` is user-controlled (passed through from
``litellm.video_content(variant=...)``) and was interpolated raw into
the URL query string. A value like ``thumbnail&extra=1`` would inject
additional query parameters into the upstream request — the same
class of issue this PR's path-segment encoding addresses. Wrap the
value in ``quote(value, safe="")`` so ``&`` / ``=`` / ``#`` cannot
terminate the ``variant`` value or open a new parameter.
Adds a regression test asserting that a malicious ``thumbnail&extra=1``
ends up percent-encoded in the URL, and that the legitimate
``thumbnail`` value still round-trips cleanly.
