chore(nonce): remove unused NonceManager implementation

[ZacLou]

Closes #603

This PR deletes the unused NonceManager implementation and its related tests, as requested in #603.

Changes

• Removed src/nonce/NonceManager.ts
• Removed nonce-specific test files:
  • src/tests/nonce-concurrent.test.ts
  • src/tests/nonce-manager-export.test.ts
  • src/tests/nonce-safe-acquire-backoff.test.ts
• Trimmed the NonceManager.safeAcquire section from src/tests/bug-fixes-577-572.test.ts, keeping the unrelated #577 regression tests for u64ToScVal / estimateRequiredFee.
• Removed NonceManager, NonceLock, and NonceManagerOptions exports from src/index.ts.

Verification

• npx tsc --noEmit passes
• Unit tests pass

[ZacLou]

Fixed the CI build failure: rollup.config.mjs still listed the deleted src/nonce/NonceManager.ts as an entry module, causing the Rollup step to fail in both Test and Lint & Typecheck jobs. Removed that entry. Local npm run typecheck, npm run lint, and npm test all pass; npm run build cannot be verified on this macOS host due to a native rollup binary load issue, but the missing-entry error matches the CI log exactly.