Add helper for building a buy sequence fixture with incrementing amounts

[Chucks1093]

Summary
Tests that verify supply accumulation or price monotonicity across sequential buys currently construct each buy inline. A helper that generates a sequence of buys with incrementing amounts reduces boilerplate and makes the test intent clearer.

Scope

• add a test helper that performs a defined number of buys with incrementing amounts starting from a given value
• return the resulting supply and balance state after the sequence
• use the helper in at least one existing sequential buy test

Acceptance Criteria

• the helper performs the correct number of buys with the expected incrementing amounts
• the returned state reflects the cumulative result of all buys
• at least one existing test is updated to use the helper

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

[OluwapelumiElisha]

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

Hello,

I would like to work on this issue.

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[dotmantissa]

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

Hi 👋 I'm a full-stack developer
I will love to work on this issue please assign.

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[kitWarse]

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

pls assign this to me

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