JS SDK: dead_server connection fixture flakes (OutcomeUnknownError vs ConnectionLostError) #351

Found by the test-strategy review on main edb8bc3: 1 of 3 re-runs fails. | new, found here | JS SDK unit connection fixtures > dead_server expects OutcomeUnknownError, gets ConnectionLostError; 1 of 3 re-runs of the file fails on main edb8bc3b | the fixture (packages/inputlayer-js/tests/fixtures/connection/dead_server.json, from #237) has the mock server accept one write then stall; the SDK must fail the write as outcome-unknown at its deadline, but sometimes the connection teardown wins and reports connection-lost (timing-dependent) | unreported; the JS unit suite runs only in the full suite, so no PR sees it. Fix the race in the mock or the SDK, add it to the conformance set, and run the JS suite on PRs (T-05) |