fix: guard against None score/metadata/content in Bedrock KB retrieval

[lavkeshdwivedi]

Summary

The AWS Bedrock Knowledge Base Retrieve API documents score and metadata as optional fields in each RetrievalResult. The current ExternalDatasetTestService.knowledge_retrieval() parser crashes on any result that omits either of these fields:

Location	Bug	Crash type
retrieval_result.get("score") < threshold	None < float when score absent	TypeError
retrieval_result.get("metadata").get(...)	None.get(...) when metadata absent	AttributeError
retrieval_result.get("content").get("text")	None.get(...) when content absent	AttributeError

Fix

Extract each field into a local variable with a safe fallback before use:

score = retrieval_result.get("score") or 0.0
metadata = retrieval_result.get("metadata") or {}
content = retrieval_result.get("content") or {}

Test plan

• Added test_knowledge_retrieval_should_handle_missing_score — missing score treated as 0.0, no TypeError
• Added test_knowledge_retrieval_should_handle_missing_metadata — missing metadata yields title=None, no AttributeError
• Added test_knowledge_retrieval_should_handle_missing_content — missing content yields content=None, no AttributeError
• All existing tests continue to pass