fix(migrate): tolerate nested npm overrides during bootstrap detection

[fengmk2]

detectVitePlusBootstrapPending assumes npm overrides values are strings, but npm supports nested override objects. A valid package.json such as this can throw during migration because the bootstrap detection calls string methods like .startsWith() on the override value:

{
  "overrides": {
    "vite": {
      "rollup": "..."
    }
  }
}

Expected behavior:

• Migration should not crash when overrides.vite or overrides.vitest is a nested object.
• Non-string override values should be treated as not satisfying the Vite+ override and then handled safely by the migration rewrite logic.
• Add a regression test covering a nested npm override under a managed key.

Suggested test:

Create an npm fixture with nested overrides:

{
  "devDependencies": {
    "vite-plus": "0.1.x"
  },
  "overrides": {
    "vite": {
      "rollup": "^4.0.0"
    }
  }
}

Run:

vp migrate --no-interactive

Assert that migration completes or reports an actionable migration error, but does not throw a TypeError from bootstrap detection.