fix: fetch aggregate mempool state endpoint

[tolga-tom-nook]

Summary

Fixes getMempoolState() so it calls the aggregate mempool state endpoint instead of the paginated mempool transaction-list endpoint.

Before this change, getMempoolState() built this URL:

/mempool/transactions?page=...

That returns a transaction list shape (count, page, pages, list) rather than the mempool state shape. This PR switches the method to:

/mempool/state

and strengthens the regression test so it awaits the SDK call and asserts state fields (inputs, outputs, transactions) instead of only checking that a Promise object exists.

RED / GREEN validation

RED proof before the code fix, after strengthening the test:

npx jest src/__tests__/mempool.test.ts --runInBand --no-cache --coverage=false --testNamePattern 'should fetch mempool state'

Failed because the method returned the transaction-list object and did not contain inputs.

GREEN after the fix:

npx jest src/__tests__/mempool.test.ts --runInBand --no-cache --coverage=false --testNamePattern 'should fetch mempool state'

Result:

PASS src/__tests__/mempool.test.ts
Tests: 1 passed, 3 skipped, 4 total

Broader validation:

npx jest src/__tests__/mempool.test.ts --runInBand --no-cache --coverage=false
npx jest --runInBand --no-cache --coverage=false
npm run build
git diff --check

Results:

PASS src/__tests__/mempool.test.ts
Tests: 4 passed, 4 total

PASS src/__tests__/blockchain.test.ts
PASS src/__tests__/mempool.test.ts
PASS src/__tests__/address.test.ts
PASS src/__tests__/transaction.test.ts
PASS src/__tests__/api.test.ts
Test Suites: 5 passed, 5 total
Tests: 22 passed, 22 total

npm run build: passed
git diff --check: clean

Note: targeted ESLint on this repo currently reports pre-existing functional/class-rule errors in src/classes/Mempool.ts and a parser project inclusion error for src/__tests__/mempool.test.ts; these are unrelated to this endpoint change. The focused Jest regression, full Jest suite, build, and diff check pass.

Bounty context: Bitgesell PR bounty/improvement program.

[MyTH-zyxeon]

Review assist for maintainers:

This looks like a focused SDK endpoint correction: getMempoolState() now calls the aggregate /mempool/state endpoint instead of the paginated /mempool/transactions query, and the test now awaits the async call and checks for the aggregate shape (inputs, outputs, transactions). I checked the patch scope and did not see wallet, key, transaction signing, or bridge-runtime behavior changes.

Suggested merge checks:

• Run the mempool Jest test with the existing SDK test harness and confirm it no longer passes accidentally through an un-awaited Promise.
• Confirm the public API contract change is intentional: getMempoolState() no longer accepts limit, order, fromTimestamp, or page, because those belong to transaction-list retrieval rather than aggregate state.
• Verify against the documented/indexed API surface that /mempool/state returns the three aggregate fields asserted here, and keep /mempool/transactions coverage in a separate transaction-list method/test if needed.

Main acceptance criterion: aggregate mempool-state callers should hit /mempool/state deterministically, with async tests proving the returned shape rather than only checking that a Promise object exists.