Add unit tests for transfer holder count update when sender balance reaches zero

[Chucks1093]

Summary

When a sender transfers their entire key balance their wallet should be removed from the holder count. When a recipient receives keys for the first time their wallet should be added to the holder count. Both cases need unit tests.

Scope

• Test sender transferring full balance: holder count decrements
• Test recipient receiving keys for the first time: holder count increments
• Test partial transfer where sender still holds keys: holder count unchanged

Acceptance Criteria

• Full transfer out decrements holder count
• First-time recipient increments holder count
• Partial transfer leaves holder count unchanged

ETA: 12 hours

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[N-thnI]

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