Wait for vector index to stabilize

[o-nikolas]

The Bedrock retrieve-and-generate system test creates an OpenSearch
Serverless vector index and then immediately proceeds to Bedrock
Knowledge Base ingestion. However, an acknowledged index-creation call
does not guarantee the index is ready for use: OpenSearch Serverless
materializes the index asynchronously and exposes no status API to
check on its progress. When ingestion starts before the index can
serve requests, the test fails intermittently.

Since there is no status API for the index, this change polls a
trivial match_all search against the new index as a readiness probe
(retrying TransportError up to 30 times at 5-second intervals)
before allowing downstream tasks to run, eliminating the race.

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