Add remote proxy lease backend for HarmonyOS (harmonyos-instance)

[bytemain]

Problem

The 0.20.10 proxy inventory lists booted HarmonyOS devices, and the local host provider supports open --platform harmonyos, but remote proxy lease admission rejects HarmonyOS because LeaseBackend only contains ios-simulator, ios-instance, and android-instance.

Observed from a Linux client connected to a Mac proxy:

• devices --platform harmonyos lists a booted emulator.
• open ... --platform harmonyos --device emulator fails before allocation with open requires --platform ios|android or --lease-backend.
• The local Mac (non-proxy) can open the same Harmony emulator and exposes snapshot/click/type/logs/screenshot.

Proposal

Add a first-class harmonyos-instance lease backend end-to-end: contracts/type, platform-to-backend mapping, proxy lease allocate/heartbeat/close, session runtime hints, and tests proving device selection and lease lifecycle. Preserve fail-closed behavior for unsupported providers; do not merely widen a client enum.

Acceptance should include remote proxy inventory -> open -> snapshot/input/logs -> close, with lease identity and artifact provenance. Physical OHOS remains out of scope; emulator/proxy only.

[thymikee]

Triage: valid, fully scoped enhancement. PR #2266 is the active implementation. Keep this issue open until that PR proves the requested remote proxy inventory → open → snapshot/input/logs → close flow with lease identity and artifact provenance; enum/allowlist coverage alone is not sufficient.