feat(AIC-3211): expose graph traversal path in ProviderGraphResponse

[ld-paul]

Summary

graph().invoke() already accumulates the traversal path and each node's response internally (it sends the path as $ld:ai:graph:path), but ProviderGraphResponse dropped both — so an app cannot tell which branch the router picked. This returns them.

Closes AIC-3211.

 export type ProviderGraphResponse = {
   response: string;
   usage: TokenUsage;
   judgeResults?: ProviderResponse['judgeResults'];
+  path: string[];
+  nodes?: Record<string, ProviderResponse>;
 };

• path is required: every graph execution path is knowable, including the native runners (toClaudeAgents(), toOpenAIAgents(), toLangGraph()), which already track it.
• nodes is optional: native runners delegate traversal to the framework and do not produce a ProviderResponse per node.
• Purely additive — no behavior or telemetry change.

Implementation details

Reported by Ben in #proj-turnkey-ai-sdk: he had a route badge showing which node the router picked, and had to scrap it when switching to graph().invoke().

graph().invoke()'s accumulate() already pushes node.key into path and stores nodes[node.key] = res; the return statement just omitted them. The three native-graph.ts runners already build a path array for their own telemetry and now return it.

The fine-grained alternative that exists today — resolveGraph() + def.route(), which returns {...response, next} — is documented alongside the new fields.

Verified with make code-check, make build, make typecheck, and make test (all packages pass). A new test asserts path order and per-node responses on a two-node graph.

Graph-level streaming (AIC-3210) is the other half of Ben's report and follows in a separate PR.

Link to Devin session: https://app.devin.ai/sessions/978ca9b3c9634c60abb7385a945d17bc
Requested by: @ld-paul

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