test: add unit tests for GistsService, SorobanService, IpfsService

[7udah]

Summary

Core backend services (GistsService, SorobanService, IpfsService) had no unit tests. This PR adds comprehensive unit test suites for all three, covering the primary code paths, mock/real mode switching, retry logic, and cache interaction.

Changes

• Added Backend/src/gists/gists.service.spec.ts — tests for create(), findNearby() (cache hit/miss, cursor), and findOne() (cache hit/miss, not found)
• Added Backend/src/soroban/soroban.service.spec.ts — tests for mock mode results, real mode retry exhaustion, and retry warning count
• Added Backend/src/ipfs/ipfs.service.spec.ts — tests for dev mode mock CID generation, getJson() fetch retries, non-OK HTTP responses, and successful retrieval

Testing

All 32 new tests pass:

PASS src/gists/gists.service.spec.ts
PASS src/soroban/soroban.service.spec.ts
PASS src/ipfs/ipfs.service.spec.ts

Test Suites: 3 passed
Tests:       32 passed

Closes #31