Fix ID length validation: reject IDs with wrong digit count

[dalito]

Add negative lookbehind to ID regex patterns so that e.g. an 8-digit ID is not silently accepted by a 7-digit pattern. Abort conversion when non-conforming IDs are found.

With this PR conversion fails with a meaningful message:

INFO    |Executing cmd: voc4cat convert --config _main_branch/idranges.toml --logfile outbox/voc4cat.log --outdir outbox inbox-excel-vocabs/
INFO    |Reading XLSX file: inbox-excel-vocabs\voc4cat.xlsx
ERROR   |Terminating with error: Found 19 non-conforming IRI(s) that do not match the configured ID pattern ((?<![0-9])(?P<identifier>[0-9]{7})$):
  https://w3id.org/nfdi4cat/voc4cat_00008201
  https://w3id.org/nfdi4cat/voc4cat_00008202

Closes #341