[DRAFT] fix(agent): preserve copilot-acp session context across requests

[MestreY0d4-Uninter]

Depends on #4536.
Please review/merge this after the ACP tool-call bridge lands, since this follow-up assumes that base.

I’m opening it early to reduce the risk of upstream drift while #4536 is still pending.

Summary

This PR improves the copilot-acp integration by making session behavior much closer to a native conversational backend.

Instead of rebuilding ACP state in a brittle way on every request, this change keeps the session/process alive across turns and replays only the incremental delta when the message history is an exact prefix extension.

What changed

• preserve ACP process/session across requests
• replay only new messages when the conversation history is a strict extension
• reset the ACP session when history/model changes incompatibly
• improve message serialization so assistant tool_calls, tool results, and richer message content survive the ACP bridge
• gracefully fall back when session.model.switchTo is unavailable in the current ACP runtime

Why

The tool-call bridge fixed the biggest behavioral bug, but ACP conversations still felt less native than other providers because context/session handling was weaker.

This PR improves continuity and makes ACP much more usable for multi-turn workflows.

Validation

Automated

• python3 -m py_compile agent/copilot_acp_client.py tests/agent/test_copilot_acp_client.py
• uv run --extra dev python -m pytest -o addopts='' tests/agent/test_copilot_acp_client.py -q
• Result: 5 passed

Manual

Validated with a real ACP multi-turn flow on the rebased branch:

1. first request returns OK
2. second request reuses prior context and returns CONTEXTO_OK

Notes

This PR does not claim strong model enforcement in ACP. It improves session/context behavior on top of the current runtime.

[MestreY0d4-Uninter]

Stacked follow-up to #4536.

Opened as draft to avoid upstream drift while the base ACP tool-call bridge is still pending.
Please review/merge this after #4536.

Related follow-ups in the same stack:

• [DRAFT] fix(agent): route copilot-acp through non-streaming request flow #5336 — ACP non-streaming run_agent.py integration
• [DRAFT] fix(tools): support delegation reasoning overrides for subagents #5337 — delegation reasoning overrides

[MestreY0d4-Uninter]

Closing as stale/superseded by later ACP integration changes. I re-audited this against a clean origin/main worktree: parts of the ACP/session behavior were covered by later ACP follow-ups, but this stacked draft no longer represents a clean, mergeable delta on top of current main. If any residual gap still matters, I'll re-extract it as a small focused PR against current main.