fix(http): force multipart/form-data even without files(#20322) (#20323)

api/core/workflow/nodes/http_request/executor.py

                                 files[key].append(file_tuple)
 
                     # convert files to list for httpx request
+                    # If there are no actual files, we still need to force httpx to use `multipart/form-data`.
+                    # This is achieved by inserting a harmless placeholder file that will be ignored by the server.
+                    if not files:
+                        self.files = [("__multipart_placeholder__", ("", b"", "application/octet-stream"))]
                     if files:
                         self.files = []
                         for key, file_tuples in files.items():
             raw += f"{k}: {v}\r\n"
 
         body_string = ""
-        if self.files:
+        # Only log actual files if present.
+        # '__multipart_placeholder__' is inserted to force multipart encoding but is not a real file.
+        # This prevents logging meaningless placeholder entries.
+        if self.files and not all(f[0] == "__multipart_placeholder__" for f in self.files):
             for key, (filename, content, mime_type) in self.files:
                 body_string += f"--{boundary}\r\n"
                 body_string += f'Content-Disposition: form-data; name="{key}"\r\n\r\n'

api/tests/unit_tests/core/workflow/nodes/http_request/test_http_request_executor.py

     assert "multipart/form-data" in executor.headers["Content-Type"]
     assert executor.params == []
     assert executor.json is None
-    assert executor.files is None
+    # '__multipart_placeholder__' is expected when no file inputs exist,
+    # to ensure the request is treated as multipart/form-data by the backend.
+    assert executor.files == [("__multipart_placeholder__", ("", b"", "application/octet-stream"))]
     assert executor.content is None
 
     # Check that the form data is correctly loaded in executor.data