[FEATURE] Include compaction calls in token usage #5404

Human Overview

Need to add the token usage for compaction to the token usage tracked by the Strands SDK.

SDK Language

Both / Not language-specific

Problem Statement

Model.compact() calls the model outside the agent loop, for both the client-side default and native compaction. Anthropic reports native compaction usage in usage.iterations. Neither shows up in agent metrics or traces, so users can't see what summarization costs. The 0022 evals had to add it up by hand.

Proposed Solution

• Add each compact() call's usage to the agent's accumulated usage, including usage.iterations for native compaction.
• Emit a span for each compaction call. Follow the convention in [FEATURE] Normalize token usage to one semconv-aligned convention #4618.
• Design 0022 doesn't cover usage accounting yet. Add a short section to it.

Use Case

A user compares the cost of compacting often against compacting rarely.

Additional Context

• Depends on: [FEATURE] Add an overridable Model.compact() and route summarization through it #5464, [FEATURE] Summarize through Anthropic native compaction #5401
• Design: team/designs/0022-server-side-compaction-support.md (docs(designs): server-side compaction support #5242)