test: keep stub server-manifest cache out of the repository root

[ScriptedAlchemy]

Summary

The shared Rsbuild API mock in tests/setup.ts had no context.cachePath, so the server-manifest stamp resolved under the current working directory, and every test run wrote react-router/<hash>/server-manifest.json into the repository root. One of those files was committed to main in #152.

The mock now uses Rsbuild's default node_modules/.cache, which is gitignored, and this removes the committed stub. It never shipped: the package only publishes dist and src.

Test-only change, so there's no changeset.

[pkg-pr-new]

Open in StackBlitz

pnpm add https://pkg.pr.new/rsbuild-plugin-react-router@b4b756c

commit: b4b756c

[github-actions]

Benchmark results

Case	Base	Head	Delta
build-256-ssr	2045.8 ms	2047.3 ms	+0.1%
dev-48-ssr	908.4 ms	883.5 ms	-2.7%

[codspeed]

Merging this PR will not alter performance

✅ 1 untouched benchmark
⏩ 2 skipped benchmarks¹

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_{Comparing fix/test-stub-cache-path (b4b756c) with main (c635929)²}

Footnotes

1. 2 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on main (66863c3) during the generation of this report, so c635929 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩