[codex] Preserve live tool history

[nick-pape]

Summary

• preserve assistant tool-call parts and tool result messages in active session model history
• keep ACP tool updates and durable event shapes unchanged
• add multi-turn coverage for successful and failed tool results

Closes #15.

Detailed validation

• .\common\temp\pnpm-local\node_modules\.bin\pnpm.cmd --dir packages/agent-core test
• .\common\temp\pnpm-local\node_modules\.bin\pnpm.cmd --dir packages/acp-agent test
• node common/scripts/install-run-rush.js build
• node common/scripts/install-run-rush.js test

[Copilot]

Pull request overview

This PR updates the agent loop so that tool-call and tool-result information is preserved in the in-memory session.history across turns, allowing subsequent model turns to receive the same tool context that occurred earlier in the session. It also adds multi-turn test coverage to ensure tool history is included in later model requests.

Changes:

• Record assistant tool-call parts and tool-result messages into session.history during streaming turns.
• Split assistant history into (a) structured parts for tool calls and (b) trailing plain assistant text, while keeping durable event persistence unchanged.
• Add multi-turn tests to assert that prior tool calls/results (success + failure) are included in later model turns.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
packages/agent-core/src/agent.ts	Adds tool-call/tool-result entries to session.history and helper logic to flush/append them during a turn.
packages/acp-agent/src/agent-prompt-cancellation.test.ts	Adds regression tests asserting prior tool calls/results are present in later model turn messages.

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