fix: correctly create multipart requests for is activities

[andreitava-uip]

• IS activities with multipart/form-data content type expect the "body" to be added as a part.
• With httpx, we were doing this by adding the body to the "files" array, because that is the only way to coerce httpx to construct multipart. This was causing httpx to also add filename: body to Content-Disposition, which caused some providers to interpret it as a file. (MS API did not work, Google API worked).

Wrong request body:

--boundary
Content-Disposition: form-data; name="body"; filename="body"
Content-Type: application/json

...body...

--boundary--

Correct request body (no filename):

--boundary
Content-Disposition: form-data; name="body";
Content-Type: application/json

...body...

--boundary--

• the fix is to pass an empty string as the first tuple param, which will omit the filename field and produce the correct body