Context summarization instead of middle-out

[SummarizingConversationManager](https://strandsagents.com/latest/documentation/docs/user-guide/concepts/agents/conversation-management/#summarizingconversationmanager)

[OpenHands Context Condensensation for More Efficient AI Agents](https://www.all-hands.dev/blog/openhands-context-condensensation-for-more-efficient-ai-agents)

The issue names no repository files, tests, or entry points; begin by reading the linked SummarizingConversationManager and OpenHands context-condensation references, then locate the current middle-out implementation in the repository. Before starting, clarify the intended scope, integration point, and acceptance criteria for replacing the existing approach.

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