test: add coverage for contract, federation, hidden streams, and cache

[Hahfyeex]

Summary

• Add mock-mode unit coverage for contract simulation, creation, batch creation, withdrawal, cancellation, lookup, and archived IDs.
• Add Federation success, malformed input, missing record, and invalid account coverage.
• Add hidden-stream/block-sender persistence, idempotency, subscription, malformed storage, and SSR behavior coverage.
• Add streams-cache bigint round-trip, timestamps/staleness, corruption, and unavailable-storage coverage.
• Repair the repository's truncated webhook hook suffix and update the Stellar SDK resource accessor so the affected source remains parseable with the installed dependencies.

Validation

• pnpm exec vitest run --coverage.enabled=false __tests__/lib/contract.test.ts __tests__/lib/federation.test.ts __tests__/lib/hidden-streams.test.ts __tests__/lib/streams-cache.test.ts — 17 passed.
• Full suite: 1,231 passed; 28 pre-existing UI failures remain outside this change.
• TypeScript still reports existing diagnostics in unrelated tests.

Closes #731
Closes #732
Closes #733
Closes #735

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