SimpleUrl Harvester may crash if json object contain only decimals

[f-necas]

Describe the bug

When creating an harvester pointing to https://www.data.gouv.fr/api/1/datasets/?organization=5c812a16634f416583ed1876&page_size=1 it crash because of json structure like:

followers_by_months": {
          "2024-08": 0,
...
          "2025-08": 0
},

ending up in xml

<followers_by_months>
            <2025-05>0</2025-05>
...
            <2024-12>0</2024-12>
            <2025-02>0</2025-02>
</followers_by_months>

which isn't valid, because xml tags :

• Start with a letter or underscore (_)
• Can contain letters, digits, hyphens (-), underscores (_), and periods (.)
• Cannot start with a number

Maybe in https://github.com/geonetwork/core-geonetwork/blob/main/harvesters/src/main/java/org/fao/geonet/kernel/harvest/harvester/simpleurl/Harvester.java#L370

It could be added something like .replaceAll("<(/?)([\\d-]*)>", "<$1_$2>");

To Reproduce
Steps to reproduce the behavior:

1. Go to Harvesters
2. Url https://www.data.gouv.fr/api/1/datasets/?organization=5c812a16634f416583ed1876&page_size=1
3. Loop /data
4. Id /id

Expected behavior
A metadata harvested

Log file
Error on line 1