feat!: Add invokeStructuredModel method to support new Judge online evals

[jsonbailey]

Note

Adds invokeStructuredModel with structured output support and wraps invokeModel with error handling; updates tests and aligns SDK versions.

• LangChainProvider (src/LangChainProvider.ts):
  • Structured Output: Add invokeStructuredModel(messages, responseStructure) using withStructuredOutput(...), returning StructuredResponse with serialized raw response and zeroed usage metrics.
  • Error Handling: Wrap invokeModel and invokeStructuredModel in try/catch to return success=false on failures and emit warnings; maintain assistant message shape on errors.
  • Types: Import StructuredResponse.
• Tests (__tests__/LangChainProvider.test.ts):
  • Add tests for invokeModel error case and for invokeStructuredModel success/error paths, including metrics and logging assertions.
• Dependencies (package.json):
  • Align @launchdarkly/server-sdk-ai dev/peer versions to ^0.12.0.

^{Written by Cursor Bugbot for commit 4c38dcc. This will update automatically on new commits. Configure here.}

[jsonbailey]

This PR is reliant on #969 being merged first.

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[cursor]

Bug: Mismatched Logging Levels for Model Invocation Errors

The invokeModel and invokeStructuredModel methods in LangChainProvider.ts log caught errors using logger.warn(), but their corresponding tests expect logger.error(). This mismatch causes test failures and suggests an incorrect severity classification for these error conditions.

Additional Locations (3)

• packages/ai-providers/server-ai-langchain/src/LangChainProvider.ts#L82-L83
• packages/ai-providers/server-ai-langchain/src/LangChainProvider.ts#L128-L129
• packages/ai-providers/server-ai-langchain/__tests__/LangChainProvider.test.ts#L168-L169