agent: harden conformance delegate retry

[asim]

Summary

• Extend provider conformance retries to give stubborn providers one final guarded-delegate attempt.
• Add a final tagged-tool-call retry prompt so AtlasCloud/minimax-compatible providers can reliably exercise the guarded delegate path before the harness fails.
• Add deterministic coverage for the final tagged delegate retry prompt.

Closes #4206
Closes #4210

Testing

• go build ./...
• go test ./... (fails in this environment because loopback gRPC targets are blocked by envoy: client/grpc and transport/grpc)
• go test ./agent -run 'TestAgentProviderConformance(Final|Retry|Fails|Atlas|Executes)' -count=1
• golangci-lint run ./...