Fix: prevent VertexAiRagRetrieval from blocking event loop

[loic-combis]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):
#5033

• Closes: VertexAiRagRetrieval.run_async() blocks the event loop with synchronous gRPC call #5033 5033

Problem:
run_async makes a synchronous gRPC call.

Solution:
Defer call to thread with asyncio.to_thread

Testing Plan

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Unit Tests:

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Hello @loic-combis, thank you for your contribution!

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