Track HarnessDriverClient client.ts coverage debt #1060

Context

During relay#1055 coverage triage, importing the new broker spawn-config helper from packages/harness-driver/src/client.ts pulled the full client.ts module into the Vitest/V8 coverage report. That exposed very low coverage for the large client module and dropped global thresholds, even though the new #125 behavior was tested.

The #1055 fix moved the new workspace-key/broker-name resolution helper into packages/harness-driver/src/spawn-config.ts, with focused tests covering the new contract. That keeps #1055 scoped, but the underlying client.ts blind spot remains.

Follow-up

Add meaningful coverage for packages/harness-driver/src/client.ts, prioritizing:

• HarnessDriverClient.spawn() behavior around broker process startup and connection setup.
• lifecycle/event-bus behavior that is currently only indirectly exercised.
• HTTP/transport wrapper paths that can be tested with fake fetch/process seams.

This is non-blocking for #1055; it is tracking pre-existing coverage debt surfaced by the PR.