Use a regular expression to extract the streams from variables files.

`CMEW/app/configure_standardise/bin/create_request_file.py` the stream is currently extracted with

```
# Get the whole stream including substream
whole_stream = line.strip().split(":")[-1]

# But don't list the substream
stream = whole_stream.split("/")[0]
```

but this could be done more efficiently with a regular expression.

Read CMEW/app/configure_standardise/bin/create_request_file.py and inspect the current stream extraction around the shown code. Replace the split-based extraction with a regular-expression approach while preserving the existing behavior of keeping the main stream and excluding its substream. Verify the script still produces the expected request-file stream values.

Written by the indexing model from the issue text.