Enforce idempotent plan-delegate notifications

[asim]

Summary:

• Cache successful delegate results by normalized target/task so replayed provider delegate calls reuse the prior result instead of re-invoking the delegated agent.
• Add deterministic duplicate-delegate harness coverage alongside existing duplicate-notify coverage.

Testing:

• go build ./...
• go test ./... (fails in this environment: loopback targets forbidden in grpc transport/client tests)
• golangci-lint run ./...

Closes #4189
Closes #4199