docs(experiments): add Node.js multirun and record tag examples

[mehulsonowal]

Summary

Adds runnable Node.js experiments examples covering the dd-trace-js experiments API work:

• experiment({ runs: 2 })
• .run({ concurrency: 2 })
• sequential run execution
• bounded task concurrency inside each run
• bounded row evaluator concurrency inside each run
• bounded summary evaluator concurrency inside each run
• nested workflow/task spans inside each multirun experiment row
• first-run result aliases via result.rows and result.summaryEvaluations
• dataset record tags, tag updates, tagged pulls, and an experiment over a tagged dataset slice in the existing dataset operations script

Also updates the Node.js examples package scripts and README so npm run validate:all covers the dataset operations and experiment examples.

Validation

• node --check passed for all files under experiments/nodejs/examples/ and experiments/nodejs/examples/lib/
• User validated npm run multirun against the dd-trace-js changes locally
• Added follow-up commits:
  • docs(experiments): make Node.js multirun example multispan
  • docs(experiments): add Node.js record tags example
  • docs(experiments): fold record tags into dataset example

Notes

This depends on dd-trace-js SDK changes in:

• feat(llmobs): add experiment multirun concurrency dd-trace-js#9686
• feat(llmobs): add dataset record tags dd-trace-js#9707