test(core): cover EntityRoot mergeFrom changes and metadata

[philipgreat]

Background

EntityRoot.mergeFrom is used when an entity graph combines independently tracked related entities. A merge error can silently lose writes or metadata.

Scope

Add focused tests for merging one root into another.

Test cases

• mergeFrom(null) is a no-op.
• Field changes from the source root appear in the target root.
• Existing target changes for unrelated keys remain intact.
• Source deleted and new keys are copied.
• Source trace chains and original versions are copied.
• Source and target roots remain independent objects after the merge.
• Source root collections remain unchanged by the merge.

Acceptance criteria

• Tests verify all five merged data categories: changes, deleted keys, new keys, trace chains, and original versions.
• No TeaQLRuntime fixture is used.