CI: switch to Yarn Classic with cache and use yarn commands for install/test

[hungvminh]

Motivation

• Ensure CI uses Yarn Classic and benefits from dependency caching to produce deterministic installs for the server workspace and to run the Gemini compatibility tests with the repository lockfile.

Description

• Configure actions/setup-node with cache: yarn and cache-dependency-path: server/yarn.lock, install Yarn Classic with npm install --global yarn@1.22.22, replace npm install with yarn install --frozen-lockfile, and run tests with yarn test:gemini-compat.

Testing

• No automated tests were executed during this change; the CI workflow is updated to run yarn install --frozen-lockfile and yarn test:gemini-compat when the pipeline runs.

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Codex Task