feat(runnable-rails): stream metadata in RunnableRails output

[Pouyanpi]

Enhance streaming in RunnableRails to include generation metadata in streamed chunks. Skips END_OF_STREAM markers and updates chunk formatting to support metadata for AIMessageChunk outputs. This improves compatibility with consumers expecting metadata in streaming responses.

[Copilot]

Pull Request Overview

This PR enhances the RunnableRails streaming functionality to include generation metadata in streamed chunks and improves compatibility with consumers expecting metadata in streaming responses. The changes focus on filtering out END_OF_STREAM markers and updating chunk formatting to properly support metadata for AIMessageChunk outputs.

• Updates streaming methods to include generation metadata in chunks
• Adds logic to skip END_OF_STREAM markers during streaming
• Enhances chunk formatting to handle metadata alongside text content

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
tests/runnable_rails/test_streaming.py	Removes outdated comments and adds new tests for metadata preservation and chunk type validation
tests/runnable_rails/test_metadata.py	Adds comprehensive test coverage for streaming metadata preservation in both sync and async scenarios
nemoguardrails/integrations/langchain/runnable_rails.py	Updates streaming implementation to handle metadata and filter END_OF_STREAM markers

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