test: stabilize JavaScript dependency install

[DaleSeo]

Motivation and Context

The JavaScript integration tests run at the same time, and each one calls npm install on the same node_modules directory. This can lead to overlapping writes, which might leave dependencies like Zod partially installed. As a result, unrelated tests could fail during module loading, as shown in https://github.com/modelcontextprotocol/rust-sdk/actions/runs/28545693359/job/84630290801. This issue has come up occasionally in PRs as well. This PR addresses it by installing the shared dependencies once for each test process and reporting a failed npm exit status instead of proceeding with a broken installation.

How Has This Been Tested?

cargo +nightly fmt --all -- --check

Breaking Changes

None.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed