[FEATURE] Add compaction to golang ADK

[EItanya]

📋 Prerequisites

• I have searched the existing issues to avoid creating a duplicate
• By submitting this issue, you agree to follow our Code of Conduct

📝 Feature Summary

Add auto compaction support to golang declarative and BYO agents

❓ Problem Statement / Motivation

Compaction is a key part of long-running agentic workflows

💡 Proposed Solution

Add it to our libraries if possible, if not contribute upstream

🔄 Alternatives Considered

No response

🎯 Affected Service(s)

None

📚 Additional Context

No response

🙋 Are you willing to contribute?

• I am willing to submit a PR for this feature