agent: continue unfinished plan steps

[asim]

Summary:

• Continue checkpointed agent runs when an intermediate model reply leaves plan steps unfinished.
• Add a regression test covering a continuation that completes exactly one delegated comms notification.

Testing:

• go build ./...
• go test ./agent -run TestCheckpointContinuesRunWithUnfinishedPlanStep -count=1
• go test ./... (fails in this environment because loopback gRPC targets are blocked by envoy)
• golangci-lint run ./...

Closes #3699
Closes #3705