Return entity edge reference time

[sakshichitnis27]

Summary

Fixes EntityEdge.reference_time being lost when edges are read from the database.

Although reference_time is persisted on entity edges, it was omitted from the
Cypher return projections. As a result, edge hydration always received a missing
value and set reference_time to None.

This PR:

• Adds reference_time to the shared entity-edge return projection.
• Adds it to the standalone Neptune full-text, similarity, and BFS search
  projections.
• Adds regression tests for query projection and edge hydration.

Type of Change

• Bug fix
• New feature
• Performance improvement
• Documentation/Tests

Objective

Ensure persisted EntityEdge.reference_time values are preserved across standard
edge reads and search results.

Testing

• Unit tests added/updated
• Integration tests added/updated
• All existing tests pass

Regression tests verify that:

• Entity-edge return queries select reference_time for every graph provider.
• Both entity-edge hydration paths preserve the returned timestamp.

The full test suite was not run locally due to environment limitations.

Breaking Changes

• This PR contains breaking changes

No breaking changes.

Checklist

• Code follows project style guidelines (make lint passes)
• Self-review completed
• Documentation updated where necessary
• No secrets or sensitive information committed

No documentation changes were necessary because this restores the existing model
field’s expected read behavior.

Related Issues

Closes #1661

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[sakshichitnis27]

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