fix(meshcore): handle disabled export private key

[robekl]

Bug

exportPrivateKey() mishandles the firmware's disabled response. The protocol parser emits .disabled, but the current request matcher treated that as nil, which means "ignore this event and keep waiting". When key export is disabled on-device, the call times out instead of surfacing the actual disabled state.

Strategy

Keep the existing async throws -> Data API and make disabled an explicit error outcome, matching the existing style used for other feature-gated commands. That avoids widening the API to Data?, keeps the change narrow, and makes the failure mode consistent with the rest of the session layer.

Implementation

• route exportPrivateKey() through the error-aware request path
• map .disabled to MeshCoreError.featureDisabled instead of ignoring it
• align the doc comment with the actual throwing behavior
• add regression tests for a real disabled response and for unrelated disabled events not satisfying other requests

Verification

• swift test --skip-build --filter MeshCoreSessionCommandCorrelationTests
• swift test in MeshCore

[Avi0n]

Thanks!