Backend/src/services/soroban service.ts

[extolkom]

closes #807

Summary
tests/soroban.service.test.ts previously only covered isStale, leaving the fund-handling and chain-decoding logic in sorobanService.ts untested. This PR adds unit tests for the remaining critical paths.
Changes

submitContractCall: added tests for the simulation-error branch and the sendTransaction ERROR status branch, using a mocked rpc.Server so no live network calls are made.
getStreamFromChain / getClaimableFromChain: added happy-path tests asserting correct decoding of chain data, plus tests confirming both functions return null (rather than throwing) when XDR decoding fails.
decodeI128 / decodeAddress: added tests covering positive and negative i128 values (including boundary/sign-bit cases) and both account (G...) and contract (C...) address decoding.
topUpStream: added a test asserting it throws when KEEPER_SECRET_KEY is unset in the environment, guarding against accidental deploys without the keeper key configured.

Testing
All new tests run against mocked rpc.Server responses, no testnet/mainnet calls are made, and the existing isStale test suite is unaffected. Ran the full suite locally — all green.
Files touched

backend/tests/soroban.service.test.ts
backend/src/services/sorobanService.ts

[ogazboiz]

heads up: main's ci was broken (the Backend CI and Backend Docker Image CI jobs) until the fixes in #969 and #974 just landed, so the red backend/docker checks on this pr are almost certainly stale, they ran against the broken main. please rebase to re-test against the now-green main: git fetch origin && git rebase origin/main && git push --force-with-lease. once it's green i'll review and merge. (if a non-backend check like Frontend CI is still red after the rebase, that part is a real issue worth a look, since frontend ci was passing on main.)