docs: clarify summary_ratio summarizes oldest messages

[ded-furby]

Closes strands-agents/harness-sdk#2575

Summary

• clarify that summary_ratio applies to the oldest messages during context reduction
• note that those oldest messages are summarized and replaced rather than implying an arbitrary slice
• align the nearby Python example comment with the same behavior

Testing

• npx prettier --check src/content/docs/user-guide/concepts/agents/conversation-management.mdx
• npm run build
  • build reaches the repo's existing broken-link checker and fails on many pre-existing unresolved API doc links unrelated to this change (for example @api/python/strands.agent.conversation_manager.summarizing_conversation_manager#SummarizingConversationManager and other existing @api/... targets)
  • no new content-specific error was introduced by this wording change

[strands-agent]

This repository has been merged into the strands-agents/harness-sdk monorepo and will be archived shortly. All new development happens there.

If this PR is still relevant, please recreate it against the monorepo. The code now lives under site/. Full commit history was preserved, so your base should be findable.

Apologies for the disruption, and thank you for contributing!