feat: add Amazon Bedrock Knowledge Base tool

[PVidyadhar]

feat: add Amazon Bedrock Knowledge Base tool

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Link to Issue or Description of Change

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

N/A — new feature.

2. Or, if no issue exists, describe the change:

Problem:
ADK agents currently have no built-in way to retrieve context from Amazon Bedrock Knowledge Bases, requiring users to write custom tool implementations for RAG workflows.

Solution:
Created BedrockKBTool(BaseTool) subclass that integrates natively with the ADK agent framework. The tool uses _get_declaration() for schema registration and async run_async() for execution, matching the standard ADK tool pattern. Supports both managed and vector KB types with agentic retrieval (query decomposition + managed reranking) and automatic fallback.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

pytest tests/test_bedrock_kb_tool.py — all tests pass with mocked boto3 calls.

Manual End-to-End (E2E) Tests:

Called tool.run_async(args={'query': '...'}) against a managed KB in us-west-2. Returned 3 results with content and scores. Called exactly as ADK agent framework would invoke it.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

• Required IAM permissions: bedrock:Retrieve, bedrock:AgenticRetrieve
• SDK requirement: boto3 >= 1.43
• Google CLA: Approved (P469013198)

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[adk-bot]

Response from ADK Triaging Agent

Hello @PVidyadhar, thank you for creating this PR! It's a great addition to the ADK tools library!

To help us move forward with reviewing your pull request, could you please address the following items from our contribution guidelines:

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