feat: add mock Horizon and Soroban RPC server for tests

[Anichris-koded]

Related issue

Closes #34

Summary

Adds src/testing/mock-server.ts, a reusable fake Horizon + Soroban RPC server, so tests stop hand-rolling fetch mocks with invented response shapes that drift from what the real servers return.

Changes made

• src/testing/mock-server.ts — createMockServer() returns a drop-in fetch implementation that serves:
  • GET /accounts/:id → a realistic Horizon account record (the shape the account loader consumes)
  • POST / (JSON-RPC) → getHealth, getNetwork, getLatestLedger, getEvents with the three Vero contract events (reg, wt_vote, resolved) whose topic XDR decodes through the SDK's own event decoder
  • anything else → a Horizon problem-details 404
• Scriptable failures via failNext(): timeouts (rejects with AbortError when the caller's signal fires, mirroring RpcClient timeouts), network rejections, 5xx, and 200s with malformed bodies. Custom routes via handle(), plus a request log for call-count assertions.
• Fixtures (horizonAccountFixture, sorobanContractEvent, sorobanEventsResult, scValSymbol, …) based on real Horizon/Soroban response shapes.
• @vero-protocol/sdk/testing subpath export in package.json; the mock is deliberately not re-exported from src/index.ts, so it never enters the main bundle (npm run size: 2.5 kB, unchanged budget 5 kB).
• Migrated src/__tests__/rpc.test.ts and src/account/__tests__/loader.test.ts onto the mock server with no behaviour change, demonstrating the acceptance criterion.
• Added src/testing/__tests__/mock-server.test.ts and extended the package smoke test to verify the testing subpath resolves in both CJS and ESM.

Testing performed

• npm test — 186 tests pass (12 suites)
• npm run test:coverage — thresholds met; mock-server.ts at 95%+ lines
• npm run typecheck — no errors
• npm run lint — no errors
• npm run build — CJS + ESM + declarations
• npm run test:package — main entry + testing subpath resolve in both formats
• npm run size — 2.5 kB, within the 5 kB budget
• npm run docs — TypeDoc generates without errors

Checklist

• npm test passes
• npm run typecheck reports no errors
• npm run lint reports no errors
• PR description links the resolved issue with Closes #<issue-number>

[N-i-xx]

This has gone into conflict with main since it was opened — could you rebase onto main and resolve? Checks were green otherwise.