perf(server): stop caching unused OpenCode tool parts (#9738)

Do not retain OpenCode tool parts after emitting their runtime events. The remaining cache readers need text, reasoning, or step-usage parts, not tool input and output.

Keep tool lifecycle events, output bytes, late-role assistant text, and usage handling unchanged. Add focused lifecycle coverage and verify retained memory through the real adapter.

Created with GPT-6 Astra (preview) in Codex.