textile plugin never validates productIdentifier

[LKSNDRTMLKV]

Found auditing plugin validate_input against schema required for every
product group (#322).

The gap

Textile v1.3.0 requires productIdentifier. TextilePlugin::validate_input
(the textile branch) checks fibreComposition, countryOfOrigin,
careInstructions and chemicalComplianceStandard. It never checks the
identifier.

Textile is the one plugin #322 did not fix by substitution, because it is the
one plugin that never called require_gtin in the first place. It was clean
through the identifier migration by accident, not by design — and that same
accident means it is now the only plugin not validating the identifier.

Note on the dual-product-group plugin

This crate serves both textile and unsold-goods, dispatched on the
in-payload productGroup discriminant. Only the textile branch should gain
this check. unsold-goods correctly has no productIdentifier — an Art. 24–25
discard disclosure covers a financial year across many products and identifies
no single one, which is why ProductGroupData::product_identifier() answers
None for it and why a registration built from one is refused.

Adding the check to the shared path would refuse every unsold-goods report.

Fix

In the else (textile) branch only:

.require_product_identifier("productIdentifier")

plus a test that an unsold-goods payload is still accepted without one — that is
the assertion that keeps the two branches honest.