Test agent retry tool side-effect dedupe

[asim]

Summary:

• Adds regression coverage that a transient provider retry reuses a checkpointed tool result instead of replaying the side effect.

Testing:

• go build ./...
• go test ./agent -run TestModelRetryDoesNotDuplicateCheckpointedToolSideEffects -count=1 -v -timeout 20s
• go test ./... (fails in this environment because loopback gRPC targets are blocked by envoy)
• golangci-lint run ./...

Closes #4175
Closes #4214