Fix partial update on simstring

[matthewmcneely]

Description

This PR fixes an issue where partial updates to structs would delete shadow similarity predicates.

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Summary by cubic

Fixes partial updates that accidentally deleted shadow similarity vectors for SimString fields. We now respect omitempty and keep existing vectors when the field is unset.

• Bug Fixes
  • Skip shadow vector deletion when a SimString is empty and tagged with omitempty; only delete when explicitly cleared or below threshold.
  • Added regression test to ensure partial updates preserve the shadow vector and avoid re-embedding.

^{Written for commit 5cf965b. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}