Async Pipeline Support

[github-actions]

Description

Add an AsyncRAGPipeline (or async_ingest / async_query methods on RAGPipeline) so the pipeline can be awaited in async web frameworks like FastAPI and Starlette.

Motivation

Synchronous pipelines block the event loop when used inside async frameworks. Async support is essential for production API services that need to handle concurrent requests without spinning up threads.

Acceptance criteria

• AsyncRAGPipeline in ragframework/pipeline/async_rag.py or async_ingest / async_query methods added to RAGPipeline
• Abstract async ABCs in ragframework/base.py: AsyncEmbedder, AsyncRetriever, AsyncGenerator — or use asyncio.to_thread to wrap sync implementations
• All existing sync implementations and tests remain unchanged
• Tests using pytest-asyncio — add it to [dev] extra in pyproject.toml
• Example: examples/async_rag.py showing usage with asyncio.run()
• CHANGELOG.md updated under [Unreleased]

Files to touch

• ragframework/pipeline/async_rag.py — new file (or extend rag.py)
• ragframework/base.py — optional: async ABCs
• pyproject.toml — add pytest-asyncio to [dev]
• tests/test_pipeline/test_async_rag.py — new test file
• examples/async_rag.py — new example

Resources

• asyncio.to_thread docs
• pytest-asyncio