[P3] Add a success-path unit test for ContractService.getDonorCountOnChain

[cyber-punk112]

Description

ContractService.getDonorCountOnChain(poolId) in src/contract/contract.service.ts parses a Soroban simulation result via scValToNative(retVal). contract.service.spec.ts's describe('getDonorCountOnChain', ...) block only tests the "returns 0 on RPC error" path — the success path, where simulateTransaction resolves with a real retval and the count is correctly extracted, is entirely untested.

Deliverables

• Add a test that stubs service['rpcServer'].simulateTransaction (via jest.spyOn, following the pattern already used for service['mapError'] in the same file) to resolve with a mock result.result.retval, and asserts getDonorCountOnChain returns the expected number
• npm run build passes

[prodbydeco]

@prodbydeco has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hello maintainer,
I have a vast experience in solving github issues.
Please do well to assign me this let me get it done for you
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[davidedde]

@davidedde has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Good morning maintainer,
I'm a full stack developer, I have studied the requirements of this project and I'll be able to deliver in record time if assigned.
Thank you

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[Annie9144]

@Annie9144 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi! After reviewing the issue, I am certain I can implement a smooth and effective solution.

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