fix(plugin): preserve V7 session defaults

[syzsunshine219]

Description

Fixes the local plugin npm release gate blocked by a timeout in the V7 episode_per_turn E2E test.

The strict test fixture replaced the complete algorithm.session object and dropped the newer bgLlmConcurrency default. That produced a NaN semaphore limit, leaving background LLM work queued until both the test and cleanup hook timed out. This change preserves all session defaults before overriding followUpMode.

No new dependencies.

Related Issue: N/A. Release-blocking failure: Actions run #30111697641.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Validated with Node.js v22.23.1:

• Unit Test
• Test Script Or Test Steps
• Pipeline Automated API Test

npm exec vitest run tests/e2e/v7-full-chain.e2e.test.ts -- --reporter=verbose
npm run lint
npm test
npm pack --dry-run --json

The focused V7 test now completes in 1.42s. The full suite passes 148 test files with 1,204 tests passed and 2 skipped, and the package dry run succeeds.

Checklist

• I have performed a self-review of my own code | 我已自行检查了自己的代码
• I have commented my code in hard-to-understand areas | 我已在难以理解的地方对代码进行了注释 (no new production code or complex logic)
• I have added tests that prove my fix is effective or that my feature works | 我已添加测试以证明我的修复有效或功能正常 (existing V7 E2E test reproduces and verifies the fix)
• I have created related documentation issue/PR in MemOS-Docs (if applicable) | 我已在 MemOS-Docs 中创建了相关的文档 issue/PR（如果适用） (not applicable)
• I have linked the issue to this PR (if applicable) | 我已将 issue 链接到此 PR（如果适用） (linked the failing release run above)
• I have mentioned the person who will review this PR | 我已提及将审查此 PR 的人: @hijzy

Reviewer Checklist

• closes #xxxx (N/A: no issue)
• Made sure Checks passed
• Tests have been provided

[Memtensor-AI]

🤖 Open Code Review

Target: PR #2158
Task: 9109f83a86014484
Base: main
Head: codex/fix-local-plugin-release-test-timeout

✅ OpenCodeReview: No supported files changed.

Generated by cloud-assistant via Open Code Review.

[Memtensor-AI]

✅ Automated Test Results: PASSED

All tests passed (2/2 executed, 1 skipped). memos_local_plugin/unit: 2 passed, 1 skipped. Duration: 6s

Branch: codex/fix-local-plugin-release-test-timeout