Fix step 0 being returned incorrectly for steprange parameters

[awarde96]

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[samdoolin]

In a request for multiple params

"step": [0, 3, 6],
"param": ["2t", "mx2t3"],

I think this might drop step 0 entirely(?), which might be a little surprising.
Step 0 may be valid for some parameters, but would only be returned if these are requested independently

"step": [0, 3, 6],
"param": ["2t"],

[awarde96]

Yes indeed, that is an issue.
Investigating further I have found that the index files on the server like this one maybe have an error https://data.ecmwf.int/forecasts/20251207/00z/ifs/0p25/oper/20251207000000-0h-oper-fc.index.
If you search mn2t3 for step 0 you will find the following line:

{"domain": "g", "date": "20251207", "time": "0000", "expver": "0001", "class": "od", "type": "fc", "stream": "oper", "levtype": "sfc", "step": "3", "param": "mn2t3", "_offset": 35243390, "_length": 224}

As you can see it has "step" : "3" and all other params have step 0. I think this is incorrect and either the index file should not have an entry fo mn2t3 for step 0 or it should be set to step 0 not 3.

Let me discuss internally and see if this can be resolved. If changed in the index file I dont think a change to the client is needed at all.