[PM-27855] [PM-26801] Fix unable to create Passkey for existing entry

[junyuanz1]

🎟️ Tracking

#2031

📔 Objective

Allow legacy logins to surface during passkey registration

• passkey registration and search in the autofill extension filtered out older logins because copyableFields was empty
• fallback to the decrypted login data whenever no copyable metadata exists so legacy items still qualify for autofill
• add regression coverage for the legacy case so the behavior stays locked in

[CLAassistant]

All committers have signed the CLA.

[bitwarden-bot]

Thank you for your contribution! We've added this to our internal tracking system for review.
ID: PM-27855
Link: https://bitwarden.atlassian.net/browse/PM-27855

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