[Security] GetReferenceMemo silently returns zero-value on not-found, enabling phantom state writes

Resolve #2602

This MR implements the changes described in issue #2602.

Summary

• Add ReferenceMemoExists checks in fetchMemoFromReference and trackReferenceMemoUsage to prevent phantom state writes when transactions reference non-existent reference IDs
• Without these guards, GetReferenceMemo returns a zero-value record (Height=0, Memo="") for missing references, and trackReferenceMemoUsage writes this phantom record to state via SetReferenceMemo, polluting the KV store
• Add corresponding ReferenceMemoExists method to the mock keeper in tests

Test plan

• All existing unit tests pass (make test)
• Linter passes (make lint)
• Blockchain consensus safety review: no non-deterministic patterns, no float math, no map iteration, no goroutines in consensus code
• The fix uses the existing deterministic ReferenceMemoExists KV store lookup
• Verified that fetchMemoFromReference returns empty string for non-existent references (preventing further processing)
• Verified that trackReferenceMemoUsage skips writing for non-existent references (preventing phantom state writes)

🤖 Generated with Claude Code