refactor: drop unused explicit_did param in resolve_identifier

- Remove _explicit_did parameter from resolve_identifier function definition
- Update call site to ignore explicit_did with pattern _ since mismatch is checked separately
- Clarify that resolved vs explicit DID mismatch is checked in resolved_did_matches_flag, not here

Fixes M-NEW-3: Removes unused parameter that was never referenced in function body

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>