feat(dsl,nodes): support agent_strategy plugin invocation

[BenjaminX]

Summary

Implements the two integration surfaces flagged in RFC #102 so external integrators can drive Dify chatflows containing agent nodes through graphon.dsl.loads.

• PR-A — dsl/slim learns the invoke_agent_strategy slim action via a new SlimAgentStrategyClient. Reuses ToolRuntimeMessage as the wire DTO because the Dify Plugin SDK defines AgentInvokeMessage(InvokeMessage): pass — wire format is identical, the tool-side decoder is shared.
• PR-B — Adds AgentNode, AgentNodeRuntimeProtocol, and SlimAgentNodeRuntime; routes BuiltinNodeTypes.AGENT through SlimDslNodeFactory. Mirrors the structure of ToolNode / SlimToolNodeRuntime.
• Example — Adds examples/chatflow_dsl_runner, a canonical downstream-integration pattern using graphon.dsl.loads against a real Dify Studio chatflow export. Two fixtures bundled: one runs out of the box, one documents a known limitation (see below).
• Chore — Bumps the pinned openai plugin in examples/slim_llm from 0.3.8 to 0.4.0 to align with the new chatflow example.

What's new

• src/graphon/dsl/slim/agent.py — SlimAgentStrategyClient, SlimAgentStrategyError, AgentRuntimeMessage alias, SlimActionInvoker DI seam.
• src/graphon/dsl/agent_runtime.py — SlimAgentNodeRuntime (AgentNodeRuntimeProtocol), mirrors SlimToolNodeRuntime.
• src/graphon/dsl/_provider.py — extracted shared canonical_vendor helper (deduped from node_factory.py).
• src/graphon/nodes/agent/{agent_node.py, entities.py, exc.py} — AgentNode, AgentNodeData (typed-wrapper agent_parameters), AgentParameterValue, AgentNodeError.
• src/graphon/nodes/runtime.py — new AgentNodeRuntimeProtocol.
• src/graphon/dsl/{node_factory.py, importer.py} — route BuiltinNodeTypes.AGENT; add to _SUPPORTED_DEFAULT_FACTORY_NODES.
• examples/chatflow_dsl_runner/ — main.py (CLI with inspect() / loads() / event streaming), chatflow_dsl_simple.yml (runnable), chatflow_dsl_agent.yml (reference fixture, see limitations), README.md, credentials.example.json.

Out of scope (deliberate)

tenant_id / user_id are intentionally not propagated through the slim runtime. When an agent strategy plugin does self.session.model.llm.invoke(...), the daemon currently performs backwards-invocation against the Dify Server inner API at :5001, which requires a tenant context. Teaching graphon to forward tenant_id / user_id would leak Dify's business-side identity model into a tenant-agnostic execution engine.

The correct long-term fix is the daemon redesign outlined in the RFC #102 "long-term direction" — let plugin code perform nested invocations without depending on Dify Server's inner API. Until that lands, chatflow_dsl_agent.yml ships as a structural reference (this is what a real Dify Studio export of a chatflow with an agent looks like) but cannot run end-to-end from a graphon-only runner. chatflow_dsl_simple.yml is the runnable out-of-box fixture.

This trade-off is documented in examples/chatflow_dsl_runner/README.md#architecture--limitations.

Test plan

• pytest tests/ — 348 passed (28 new tests across test_slim_agent.py, test_agent_node.py, test_slim_agent_node_runtime.py, test_node_factory_agent.py).
• make tc — ruff format + check + ty all green.
• Local end-to-end: python3 examples/chatflow_dsl_runner/main.py chatflow_dsl_simple.yml "Say hi in 5 words." streams a real LLM response from langgenius/openai:0.4.0.
• CI run on this PR. Unit tests use a fake action_invoker injection seam — no e2e slim-binary tests on CI by design.

Open questions for reviewers

1. PR scope — RFC RFC: Slim runtime support for agent_strategy and tool plugin invocation #102 originally split the work into PR-A (slim) and PR-B (node). They're bundled here because PR-B is hard to verify without PR-A and the example is the smallest end-to-end witness we have. Happy to split into stacked PRs if preferred — the three commits (984a416 slim+node, 23855bd example, f439196 slim_llm bump) are natural boundaries.
2. AgentRuntimeMessage = ToolRuntimeMessage alias — keeps a semantic name so future agent-only message variants can diverge, while structurally reusing the tool DTO and decoder. Alternative is to inline ToolRuntimeMessage at the agent call sites. The reasoning is in a comment at src/graphon/dsl/slim/agent.py:15.
3. AgentNode.version() → "1" — matches the start / llm / tool convention. Distinct from tool_node_version: "2" in AgentNodeData, which is the Dify plugin protocol version forwarded verbatim to the strategy plugin.
4. chatflow_dsl_agent.yml placement — kept under examples/ with a README disclaimer rather than tests/fixtures/ because it doubles as documentation for the architecture limitation. Open to relocation if examples/ is meant to be strictly run-out-of-box.
5. slim_llm openai bump (f439196) — kept as a separate commit; trivially droppable from this PR if maintainers prefer it as a follow-up chore.

References

• RFC: RFC: Slim runtime support for agent_strategy and tool plugin invocation #102

[github-actions]

All contributors on this pull request have signed the CLA.
_{Posted by the CLA Assistant Lite bot.}

[BenjaminX]

I have read the CLA Document and I hereby sign the CLA