Add integration test for resetIndexer/replayFromLedger racing concurr…

[Fury03]

Closes #1293

Problem Statement (The Bug)

The resetIndexer and replayFromLedger functions are tested in isolation but never interleaved with SorobanEventWorker's own mutex-protected poll cycle. This means the race described in Functional Edge Case #19 is untested: when an operator triggers incident-recovery tooling while a poll is mid-flight, the poll's stale cursor can silently overwrite the reset/replay cursor, rendering the recovery action ineffective.

This issue cannot be fixed with a local patch because it requires concurrent execution of two independently-tested code paths — something isolated unit tests structurally cannot cover.

Solution Comparison and Decision

Option	Description	Why rejected
A. Unit test with mocked concurrency	Mock both paths and assert ordering	Does not exercise real async interleaving; the race depends on actual promise resolution ordering
B. Mutex extension only (no test)	Extend the worker mutex to cover reset/replay	Addresses the symptom without proving it works; regressions go undetected
C. Integration test with deferred promise	Use a deferred getEvents mock to inject reset/replay mid-flight, assert reset cursor wins	Chosen — directly reproduces the real race, fails against current code, passes once fixed

The Change (Code modifications)

Added backend/tests/integration/reset-replay-race.test.ts — a single new test file containing two integration tests.

Core test mechanism:

// Deferred promise controls when the poll's getEvents call resolves
let resolveGetEvents!: (value: any) => void;
const getEventsDeferred = new Promise((resolve) => { resolveGetEvents = resolve; });

// Shared mutable state simulates Postgres — both worker and reset/replay write here
let dbIndexerState = { lastLedger: 199, lastTimestamp: "2024-01-01T00:00:00Z" };

// Upgraded mock: first call blocks, second call (after reset) reads from shared state
server.getEvents
  .mockImplementationOnce(() => getEventsDeferred)
  .mockImplementationOnce(() => ({ events: [mockEvent], latestLedger: 210 }));

// Inject reset mid-flight while poll is awaiting RPC
resetIndexer(50, "incident recovery");

// Release the poll — it now writes its stale cursor, overwriting 50
resolveGetEvents({ events: [mockEvent], latestLedger: 210 });

Test 1 — resetIndexer race: Starts a poll, injects resetIndexer(50) mid-flight via deferred promise, asserts reset cursor wins. Current code fails: poll overwrites 50 with stale 200.

Test 2 — replayFromLedger race: Starts a second poll mid-flight, injects replayFromLedger(100), asserts replay cursor wins. Current code fails: second poll overwrites 100 with stale 300.

Compatibility Note

No INTERFACE_VERSION change. This PR adds only a test file; no production code is modified.

Testing

Test	Current code	Expected after fix
resetIndexer cursor survives concurrent poll	FAILED (got 200, expected 50)	PASSED
replayFromLedger cursor survives concurrent poll	FAILED (got 300, expected 100)	PASSED

Pre-existing failures in indexer-state.test.ts, soroban-event-worker.test.ts, and eventRace.test.ts are unrelated.

Additional Notes

Scope: Only backend/tests/integration/reset-replay-race.test.ts is added. No production code is modified.