woocommerce evals: don't crash on unknown email charsets in Subject headers

[bleonard]

The woocommerce-product-recall verifier failed whole runs with:

Recall Emails: ❌ FAILED
  Details: Recall email check error: unknown encoding: unknown-8bit

email.header.decode_header can report charset labels that are RFC-legal but have no Python codec (e.g. unknown-8bit); calling part.decode(encoding or 'utf-8') on those raises LookupError, which fails the entire email check regardless of what the agent actually sent.

Body decoding was already hardened against exactly this (_decode_email_part falls back to utf-8 with errors="replace"); the Subject path wasn't.

This PR applies the same fallback to Subject decoding in woocommerce-product-recall, and to the identical pattern in woocommerce-stock-alert (two sites) and woocommerce-new-product.