Add unit tests for the buy function panicking with insufficient_funds when the buyer sends less XLM than the required cost

[Chucks1093]

Summary

The buy function must verify the attached XLM payment covers the full cost including fees before executing. Unit tests should confirm underpayment panics with the correct error and that the exact required amount succeeds.

Scope

• Test sending exactly the required amount (cost + fees) succeeds
• Test sending 1 stroop less than required panics with insufficient_funds
• Test sending more than required succeeds and does not refund the excess (excess handled by caller)
• Test sending 0 XLM panics with insufficient_funds
• Assert no supply change or event emitted on any panic

Acceptance Criteria

• Exact required amount succeeds
• 1 stroop short panics with insufficient_funds
• Zero XLM panics with insufficient_funds
• No supply change or event on panic

ETA: 12 hours

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Coordinate on Telegram

[devJaja]

I'm an experienced software developer, Can I handle this issue please

[grantfox-oss]

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Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #708)
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[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Third Campaign campaign!

@devJaja's PR #713 was approved and merged by @Chucks1093.

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👏 Great work, @devJaja! Keep contributing to accesslayerorg.