Add helper for computing expected key balance after mixed buy and sell sequence

[Chucks1093]

Summary
Test fixtures that assert key balances after a sequence of buys and sells inline the expected values as magic numbers. A helper that computes the expected balance from a list of buy and sell amounts makes the sequence intent clear and reduces maintenance.

Scope

• add a test helper that takes an initial balance and a sequence of buy and sell amounts and returns the expected final balance
• use the helper in at least one existing test that sequences multiple trades
• keep the helper in the test module

Acceptance Criteria

• the helper correctly computes the expected balance for any buy and sell sequence
• at least one existing test is updated to use the helper
• the helper handles mixed sequences of buys and sells in any order

Contributor coordination: if you pick this up, reach out in Telegram for updates or clarification: https://t.me/accesslayerorg

[mgtmdccix-oss]

@mgtmdccix-oss has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

hello i will like to work on this issue. kindly assign, thanks.

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[rayvorden]

@rayvorden has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

I'd like to work on this.

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