feat: add Managed Knowledge Base support to AWS Bedrock KB Retriever

[PVidyadhar]

• Added Knowledge Base Type dropdown (MANAGED/VECTOR)
• MANAGED path tries langchain first, falls back to direct Bedrock API
• Direct API uses managedSearchConfiguration
• Returns proper langchain Document objects via BaseRetriever subclass
• Unit tests included
• Added BEDROCK_MANAGED_KB.md design doc

[gemini-code-assist]

Code Review

This pull request introduces support for Amazon Bedrock Managed Knowledge Bases to the AWSBedrockKBRetriever node, allowing users to select between managed and traditional vector search. It implements a new ManagedKBRetriever class that supports agentic retrieval with query decomposition and automatic fallback mechanisms, adds corresponding unit tests, and provides documentation. The code review feedback suggests several key improvements: adding defensive nullish checks to prevent runtime errors in getSourceUri, optimizing performance by avoiding dynamic imports inside the query execution loop, and using the public getRelevantDocuments method instead of the protected _getRelevantDocuments to ensure LangChain lifecycle hooks and tracing are preserved.

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