add bounded retry-with-backoff to Soroban read calls

[adelekevictor12]

closes #436

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[adelekevictor12]

The Vitest Coverage check is failing on 18 pre-existing test failures in 5 files I didn't touch:

• tests/api/marketplace-cache-invalidation.test.ts (9 failures) — CacheKey.marketplaceStats is not defined
• tests/api/etag.test.ts (2 failures) — uses non-existent toStartWith Chai property, and a property-order assertion mismatch
• tests/api/user-preferences.test.ts (3 failures) — notification merge logic
• tests/lib/backend/contracts.timeout.test.ts (3 failures) — tests hang for 5s and time out
• tests/lib/backend/contracts_versioning.test.ts (1 failure) — expects contractVersion: 'v2'

These all fail on master as well (verified locally by running pnpm test on a clean checkout). My diff doesn't touch any of those files.

My 25 new tests in tests/api/contracts.service.retry.test.ts all pass. Happy to discuss whether to merge despite the red check or wait until those pre-existing failures are addressed by their owners.