fix(acp): make session/set_mode plan idempotent on sessions created in plan mode

[ApexYash11]

Related Issue

Resolve #3356

Problem

Over ACP, session/set_mode with modeId: "plan" fails with -32603 Internal error ("Already in plan mode") right after session/new reports currentModeId: "default". When default_plan_mode = true, the engine enters plan mode at session creation, but the ACP server's local mode state stays "default" and setMode re-enters plan unconditionally, tripping the engine's guard. ACP clients that apply a configured plan mode right after session/new abort session creation entirely.

What changed

In packages/acp-server/src/session.ts:

• Seed the local currentModeId from the engine's actual plan state during session init, so session/new truthfully reports "plan" when the engine entered plan mode at create time.
• Make setMode's plan toggle idempotent — only call enterPlan/cancelPlan when the engine's plan state differs from the requested mode — so re-asserting the active mode no longer throws "Already in plan mode".

Added an integration test in packages/acp-server/test/config.test.ts that boots with default_plan_mode = true, asserts session/new reports plan, and verifies session/set_mode "plan" succeeds.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue (external PRs: the issue must have a maintainer's /approve).
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

[changeset-bot]

🦋 Changeset detected

Latest commit: ac44034

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@moonshot-ai/kimi-code	Patch

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve plan cancellation when status reads fail

When getPlan() rejects while the engine is actually in plan mode (for example, status() hits a non-missing plan-file I/O error), returning false treats the state as inactive. That makes init() report default, and more importantly makes setMode('default' | 'auto' | 'yolo') skip cancelPlan() before updating permission/currentModeId and emitting a non-plan mode, leaving the engine in plan mode while ACP says it exited; propagate the unknown state or still issue the cancel for non-plan targets.

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