Preserve Agent Type when merging agents without auto-populate

[acwhite211]

Fixes #5072

Found a simple fix to the issue. When record merging is opened with auto-populate disabled, the merged record no longer starts from schema defaults. It now preserves unanimous values from the selected records, so merging two Person agents no longer silently changes AgentType to Organization while still leaving real conflicts available for review.

Checklist

• Self-review the PR after opening it to make sure the changes look good and
  self-explanatory (or properly documented)
• Add relevant issue to release milestone
• Add pr to documentation list

Testing instructions

Confirm it working on any test database, as well as confirming it's working on the kuaccessions_2024_07_05 database.

• In user preferences, disable Record Merging > Auto-populate.
• Build an Agent query that returns only records with AgentType = Person.
• Select two Person agents and open Merge Records.
• Verify that with Show conflicting fields only enabled, Agent Type is not shown as a conflict.
• Disable Show conflicting fields only.
• Verify that Agent Type in the merged record is still Person, not Organization.
• Complete the merge.
• Open the merged agent record.
• Verify that Agent Type is still Person.