test: add integration test for get_key_balance after buys and sells

[sojetunde8]

Closes #679

Summary

• Adds integration test verifying get_key_balance returns correct key count after a sequence of buys and sells
• Covers: buy 5 keys (balance=5), buy 3 more (balance=8), sell 4 (balance=4), sell remaining 4 (balance=0)
• Asserts a wallet that has never transacted returns 0

Testing

Ran the new integration test with cargo test --test get_balance_key_count_after_trades — 1 test passed, confirming get_key_balance returns the expected balance at each step.

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