Fix multipart/form-data boundary issue in HTTP Call node (#23903)

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

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

@@ -243,8 +243,6 @@ def test_executor_with_form_data():
     # Check the executor's data
     assert executor.method == "post"
     assert executor.url == "https://api.example.com/upload"
-    assert "Content-Type" in executor.headers
-    assert "multipart/form-data" in executor.headers["Content-Type"]
     assert executor.params is None
     assert executor.json is None
     # '__multipart_placeholder__' is expected when no file inputs exist,
@@ -252,6 +250,11 @@ def test_executor_with_form_data():
     assert executor.files == [("__multipart_placeholder__", ("", b"", "application/octet-stream"))]
     assert executor.content is None
 
+    # After fix for #23829: When placeholder files exist, Content-Type is removed
+    # to let httpx handle Content-Type and boundary automatically
+    headers = executor._assembling_headers()
+    assert "Content-Type" not in headers or "multipart/form-data" not in headers.get("Content-Type", "")
+
     # Check that the form data is correctly loaded in executor.data
     assert isinstance(executor.data, dict)
     assert "text_field" in executor.data