Ensure validate request checks branching method

During early processing for CMIP7, it appears that a request with the incorrect branching method (set to `no parent` as opposed to `standard`) had passed validation. This should be investigated and updated to ensure this type of error is flagged where possible in the future.

Start by locating the request-validation entry point and the handling of the branching method, then trace how `no parent` and `standard` are interpreted. Add or update a focused validation test showing that an incorrect branching method is rejected, and run the relevant validation test suite to confirm the request is flagged.

Written by the indexing model from the issue text.