[uche001-dev]

Summary

Investigated all four issues and found the underlying circuit-breaker, failover, simulation-cache, and union-decoding implementations already present and correct. Added the missing test coverage that each issue's acceptance criteria call for, plus one genuinely new feature (transaction size tracking) that had no prior implementation.

closes #855
closes #856
closes #857
closes #858

Changes

#855: src/network/endpointFailover.ts's EndpointPool and createFailoverFetch already implement the circuit breaker pattern the issue asks for (consecutive-failure threshold, time-based recovery window, ordered healthy-endpoint selection), but had no test for the skip-unhealthy or recovery-after-window behavior. Added src/tests/endpointFailover.test.ts cases for both, using a mocked Date.now clock to control the recovery window deterministically.
#856: src/soroban/simulateTransaction.ts already caches by a contract-call identity key (contract ID, method, args) with a configurable TTL (default 5 minutes), but had no direct test, src/tests/contractCache.test.ts covers a different function (readContract). Added src/tests/simulateTransactionCache.test.ts covering cache hits on an identical call, cache misses on a different method or contract, default and custom TTL, no-cache passthrough, and clear() forcing a fresh RPC call.
#857: src/soroban/resultDecoder.ts's enum/union branch already preserves the discriminator ({ type: variantName, value }) for both no-payload and payload variants, but the existing test only covered the Some side of an Option. Added cases for the None variant, both sides of a Result<Success, Failure> union with distinct payload shapes, and rejection of an unknown variant or a malformed (non-vector) enum value.
#858: no existing implementation tracked transaction size as operations were added. Added src/transaction/transactionSizeTracker.ts (TransactionSizeTracker class), which tracks operation count and an estimated XDR byte size, warning once per threshold crossing via a configurable logger (defaulting to console.warn) when the protocol hard limit (100 operations) or either of two conservative soft limits (50 operations, ~5KB estimated size) is crossed. Exported from src/transaction/index.ts alongside the existing public API. Did not modify TransactionBuilder or buildTransaction.ts call sites directly since wiring every existing builder usage through this tracker would be a much larger, riskier change than this issue's scope; the tracker is ready to be adopted incrementally.

Test plan

Could not run the project's real test suite or typecheck in this environment: node_modules were not installed in this clone, and the development machine is currently under heavy memory pressure (confirmed via vm_stat/top showing near-exhausted physical memory and active swapping from other work running in parallel), so a fresh npm install plus vitest/tsc run was not attempted to avoid another OOM crash.

Verified correctness by tracing every new test's assertions line by line against the actual implementation being tested (EndpointPool's cursor/recovery logic, simulateTransaction's cache-key and TTL branches, resultDecoder's enum-decoding switch, and the new TransactionSizeTracker's own threshold math), and syntax-checked every new/changed file with esbuild (type-stripping, not a full typecheck) to catch gross errors. Flagging this explicitly rather than claiming a real test run that did not happen.