fix(plugin): resolve DSH Viewer assets in clean builds

[MatthewZhuang]

Description

Fix the DSH Viewer static-root resolution exposed by the 2.0.16 release dry run. In a clean checkout, the test job runs before prepack builds viewer/dist; the previous existence-based fallback therefore selected apps/viewer/dist instead of the plugin-owned Viewer directory.

The resolver now identifies the package root through its stable package.json marker, so both source (adapters/deepseek-harness) and packed (dist/adapters/deepseek-harness) layouts resolve to <package>/viewer/dist even before Viewer assets are built. A regression test covers both clean layouts, including a package root itself named dist.

Failed dry-run evidence: https://github.com/MemTensor/MemOS/actions/runs/31927133607

Related Issue (Required): Follow-up fix for #2254

Type of change

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

How Has This Been Tested?

• Unit Test: npx vitest run tests/unit/adapters/deepseek-harness-viewer.test.ts (11 passed)
• Test Script Or Test Steps: npm run lint
• Test Script Or Test Steps: npx vitest run --silent (175 files passed; 1448 passed, 2 skipped)
• Test Script Or Test Steps: npm pack --pack-destination tmp/dsh-viewer-static-root-pack and verified the tarball contains the compiled DSH adapter plus viewer/dist/index.html

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
• 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
• Documentation update is not applicable for this packaging-path fix
• I have linked the originating PR and failed release run

Reviewer Checklist

• Made sure Checks passed
• Tests have been provided

[Memtensor-AI]

⚠️ Automated Test Results: ENV ISSUE

The test environment encountered an issue that requires manual attention.

Details: Environment preparation failed before any gating tests executed. Failed scopes: memos_local_plugin
Branch: fix/dsh-viewer-static-root