Add Stellar DEX source integration tests with mocked Horizon

[prodbycorne]

Overview

src/services/sources/stellarDex.js fetches the orderbook from Horizon to derive prices. It has no tests, so regressions in the price derivation logic (midpoint calculation, empty orderbook handling) go undetected.

Scenarios to Test

Scenario	Expected behaviour
Normal orderbook (asks + bids)	Returns midpoint of best ask and best bid
Empty asks array	Uses best bid only
Empty bids array	Uses best ask only
Both empty	Returns null
Horizon returns non-200	Throws error
Horizon times out	Throws error with timeout message
XLM/native asset (no issuer)	Uses correct orderbook pair with native

Test Setup

Mock @stellar/stellar-sdk Horizon.Server:

vi.mock('@stellar/stellar-sdk', () => ({
  Horizon: {
    Server: vi.fn().mockImplementation(() => ({
      orderbook: vi.fn().mockReturnValue({
        call: vi.fn()
      })
    }))
  }
}));

Price Derivation Verification

Given:

• Best ask: 0.12 (someone selling XLM for USDC)
• Best bid: 0.11 (someone buying XLM with USDC)

Expected midpoint: 0.115

Test should assert this calculation is correct, not just that a number is returned.

Acceptance Criteria

• All 7 scenarios above have passing tests
• Midpoint calculation asserted precisely (not just truthy)
• Empty orderbook returns null gracefully
• Horizon mock used — no real network calls
• Tests run in < 100ms
• Code coverage for stellarDex.js ≥ 90%

[YfengJ]

Hi maintainers, I would like to apply for this issue if it is still available.

If assigned, I can add integration tests for the Stellar DEX source using mocked Horizon responses, cover success and malformed/empty response cases, and keep the production source behavior unchanged.

I will wait for maintainer or GrantFox assignment/confirmation before cloning, implementing, or opening a PR.

[grantfox-oss]

🦊 GrantFox — @Jennylila has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #33)
2. Your PR will be reviewed by the SmartDropLabs maintainers

Good luck! Track your progress on GrantFox.