[MyPrototypeWhat]

This PR adds a dedicated workflow for plugin tests and documents how contributors should write them.

What changes

.github/workflows/plugin-tests.yml (new)

• Triggers on pull_request only, limited to changes under plugins/**.
• Uses permissions: contents: read. Checkout runs with persist-credentials: false.
• Finds the plugins/<author>/<plugin-id> directories the PR changes and keeps only those whose package.json has a test script. It then runs npm test inside each of those directories, on Node.js 22.
• Plugins the PR doesn't touch are never run. If no changed plugin has a test script, the workflow does nothing.

CONTRIBUTING.md / CONTRIBUTING.zh-CN.md
Adds a "Plugin tests" section:

• Plugin tests are optional and live inside the plugin's own directory. They run through that plugin's npm test.
• Tests must not use the network or depend on the MiniMax Code host app.
• Dependencies are installed only inside the plugin directory and never in the root package.json.
• The root test/ directory is reserved for repo-wide Mini App rule checks. It must not import or run plugin tests.

Why

Plugin tests stay out of repo-wide CI. The rules check (npm run check) and root test/ stay exactly as they are, so a plugin's own test suite can't slow down or break the rules CI for unrelated PRs.

Notes

• This PR doesn't change any existing workflow, root test, script, or the root package.json.
• The workflow only reads the repository and needs no secrets, so it can safely run on fork PRs.

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