Add JSDoc for basicContextPipeline getter in BasicCrawler

[Copilot]

Addresses a review request to document the basicContextPipeline getter, which was previously undocumented despite non-obvious design intent.

• JSDoc added to basicContextPipeline getter explaining:
  • Why the pipeline is split: allows subclass crawlers to rerun this portion independently on the same basic context (e.g. AdaptiveCrawler retrying with a different crawler type)
  • The basic pipeline is side-effect-free, unlike subclass pipelines (which may e.g. launch a browser)
  • Typing intent: subclass pipelines expect basic crawler fields already present in the context at runtime

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