test(hosting): pin standalone-hosting-nuxt toolchain

[sarayev]

Problem

The standalone-hosting-nuxt e2e fixture floated its entire Nuxt toolchain on caret ranges (nuxt ^4.4.5, vue ^3.5.34, vue-router ^5.0.6, @nuxt/image ^2.0.0). The test setup copies the fixture to a fresh tmpdir and runs a plain npm install, so each CI run can resolve a different nuxt / @nuxt/nitro-server / vue-bundle-renderer combination.

A drifted combination made nuxt build fail during Nitro prerender of the /about route on the windows-2025 runner:

[error] [GET] /__nuxt_error ... statusCode=500 ... Server Error
  Either manifest or precomputed data must be provided
    at createRendererContext (node_modules/vue-bundle-renderer/dist/runtime.mjs:52:39)
Errors prerendering:
  ├─ /about  [500] Server Error
[error] Exiting due to prerender errors.
[BackendBuildError] Unable to deploy due to CDK Assembly Error
  ∟ [NitroBuildError] Framework build failed.
    ∟ Command failed (exit code 1): npm run build

The build dies during CDK synth, before any deploy runs, failing the standalone_hosting_nuxt deployment e2e (stages 2/2b/2c/2d, and 3's SWR check cascades on the missing URL).

Fix

Pin the four direct deps to exact versions and add a vue-bundle-renderer override. vue-bundle-renderer is the transitive dep implicated in the prerender failure, and it is pulled with a loose range that pinning nuxt alone does not lock, so the override pins it directly. This keeps the change to a single reviewable file and relies on the exact pins for deterministic resolution, rather than committing a full lockfile.

Verification

• Pinned package.json installs and nuxt build completes successfully on Linux (prerender passes, build exits 0).
• @aws-amplify/integration-tests is private, so no changeset is required.
• The Windows-only prerender failure could not be reproduced in a Linux environment, so this needs a windows-2025 e2e run to confirm the fix. Opened as draft for that reason.

Notes

Unrelated to the backend-deployer release PR #3357 - that failure chain surfaced this pre-existing fixture flake, but the deployer change is never reached (the build fails before deploy).

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[sarayev]

Reopening from an internal branch (aws-amplify/amplify-backend) instead of my fork so the deployment e2e can obtain the AWS OIDC credentials fork PRs can't get. Same commit.