fix(blockchain): return get_total_supply rpc result + unit tests

[MikeGarciaAGM]

Summary

• Fix get_total_supply() to return the RPC result value.
• Preserve existing behavior where result may be None.
• Add focused unit tests for return-value behavior and missing-result error handling.

Notes

• Tests are pure unit tests using monkeypatch (no node endpoint required).
• Related bounty context: [BOUNTY] Improve Harmony Python SDK bounties#7

[MikeGarciaAGM]

Closing this in favor of #51 to keep review focused on one PR with the same fix scope + focused tests. Please review #51.