feat: Update LangChain runners to implement Runner protocol returning RunnerResult

[jsonbailey]

Summary

• `LangChainModelRunner` and `LangChainAgentRunner` now formally inherit from the `Runner` protocol class.
• `LangChainModelRunner.run()` implements the unified `Runner` protocol; returns `RunnerResult` with `content`, `metrics`, `raw`, and `parsed` fields. Structured output is supported via the `output_type` parameter.
• `LangChainAgentRunner.run()` updated to return `RunnerResult`; populates `tool_calls` in `LDAIMetrics` from observed tool call names in the message response.
• Legacy `invoke_model()` and `invoke_structured_model()` removed — the deprecated type definitions (`ModelRunner`, `AgentRunner`, `ModelResponse`, etc.) are kept in `server-ai` for provider CI compatibility until a cleanup PR removes them.

Stacking

Stacked on top of `jb/aic-2388/openai-runner-protocol` (PR #149).

Test plan

• `make test-langchain` — all LangChain provider tests pass
• `make test` — all tests across server-ai, langchain, and openai packages pass

🤖 Generated with Claude Code

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Missing deprecated adapter methods breaks Judge evaluations

High Severity

The PR description states that legacy invoke_model() and invoke_structured_model() are "retained as deprecated adapters that delegate to run() for backward compatibility," but both methods were completely removed from LangChainModelRunner. The Judge class at packages/sdk/server-ai/src/ldai/judge/__init__.py:79 directly calls self._model_runner.invoke_structured_model(...), and the deprecated ManagedModel.invoke() at packages/sdk/server-ai/src/ldai/managed_model.py:121 calls self._model_runner.invoke_model(...). When a LangChainModelRunner is used as the judge's model runner (which happens via RunnerFactory.create_model()), both paths will crash with AttributeError at runtime.

Additional Locations (1)

• packages/ai-providers/server-ai-langchain/src/ldai_langchain/langchain_model_runner.py#L36-L55

 

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