fix: yargs esm fix test - #8460

Recently, Node.js v24 and v26 were added to the GHA test matrix (#8445). However, the test runners (c8 and mocha) both depend on [email protected].
In Node.js >= 24, when a package specifies "type": "module" in its package.json, Node.js treats extensionless files inside that package as ES Modules (ESM). Since [email protected] contains an extensionless file named yargs that uses CommonJS require(), booting up c8 or mocha on Node >= 24 crashes immediately with:

ReferenceError: require is not defined in ES module scope, you can use import instead

We cannot simply upgrade yargs to 18.0.0 globally because yargs@18 is ESM-only, which would immediately break our test suite on Node 18 (our current minimum published version) because Node 18 does not support require(esm) by default.

The Solution

To support both Node 18 and Node 24/26 concurrently during this transitional period, we implement a dynamic pnpmfile hook:

1. Root .pnpmfile.cjs: Added a root-level readPackage hook that dynamically scans package dependencies. If it detects yargs and the running Node.js version is Node >= 24, it dynamically overrides yargs to 18.0.0 (which is fully ESM-compatible and doesn't crash). On Node 18/20/22, it leaves it at 17.7.2 to ensure backward compatibility.
2. CI Integration (ci/run_single_test.sh): Updated the test runner script to pass the --pnpmfile flag pointing to the root .pnpmfile.cjs during all package installations.

This is a zero-risk, highly-isolated, transitional solution. Once Node 18 is officially retired, this patch can be easily removed by running pnpm patch-remove yargs and upgrading to yargs@18 globally.

Related Pull Requests

• Unblocks: chore: remove stale google/cloud/common_resources.proto copies #8457
• Unblocks: chore: re-consolidate eslint rules #8458
• Unblocks: chore: re-delete mocharc file #8459