fix(sync): clear connect state when the remote aborts a dial as already-syncing

[vladimirlogachev]

Description

When our dial is rejected by the remote with AbortReason::AlreadySyncing, on_sync_via_connect_finished skips on_sync_finished, so NamespaceStates::finish never runs for that exchange. If no concurrent incoming exchange took the slot over (mutual-dial tie-break), the (namespace, peer) pair stays Running forever and every later sync trigger for it — NewNeighbor, SyncReport, Resync — is silently dropped in start_connect. The rejection is easy to hit when one side redials right after an exchange while the other side has not yet processed its finish event; downstream we observed pairs permanently stopping to sync, caught by a scenario stress test failing about once in 80 runs.

Changes:

• engine/state.rs: add NamespaceStates::abort_connect, clearing the running outgoing exchange back to Idle — guarded so it only clears a Running { origin: Connect(reason) } slot with the same SyncReason; an accept exchange that took the slot over in the tie-break is left untouched (it finishes through the accept path), as is a newer dial with a different reason.
• engine/live.rs: call it from the RemoteAbort(AlreadySyncing) arm of on_sync_via_connect_finished.
• Three unit tests on the state machine: the rejected dial returns the pair to idle; a stale abort does not clear a newer dial; an accept takeover is spared.

Breaking Changes

None — additive internal change to the live engine's sync-state tracking; no public API or wire format touched.

Notes & open questions

The guard distinguishes exchanges by SyncReason, not by session identity — a late abort of an old dial can clear a newer dial started for the same reason. The consequence is an extra idle transition (the next trigger restarts the exchange), not a wedge. If you want it stricter, SyncState::Running would need a session id; happy to extend if preferred.

Change checklist

• Self-review.
• Documentation updates following the style guide, if relevant.
• Tests if relevant.
• All breaking changes documented.