ci: build otel-plugin before tests; explicit plugin install

[mojoatomic]

This PR fixes CI failures due to @rdcp/otel-plugin not being built in fresh checkouts.

Changes:

• Add explicit steps in CI to npm ci --prefix packages/otel-plugin and npm run build --prefix packages/otel-plugin before tests
• Keep Node pinned via .nvmrc and explicit demo-app install step

Outcome:

• Ensures demo app tests can require @rdcp/otel-plugin without module resolution errors in CI.