Add and improve error messages when users supply custom functional integrals with incorrectly sized return tensors.

Seems like something easy to check at either runtime or compile time. This cost us some time debugging what should have been a non-issue.

Start by locating the custom functional integral handling and any validation or diagnostic tests for return tensors. Check how tensor sizes are currently validated at runtime or compile time. Done means incorrectly sized return tensors produce clear, actionable error messages, with coverage for the relevant failure cases.

Written by the indexing model from the issue text.