Add LangChain Search and Extract tools

[georgeatparallel]

Adds @parallel-web/langchain with Search and Extract tools and a runnable research-agent example.

The tools use the Parallel SDK and LangChain's standard tool interface. They validate inputs, preserve complete API responses as artifacts, limit the text sent to models, and pass cancellation through to the HTTP request. The example shares a search session and asks for source citations.

Validation:

• 47 package tests pass on Node 22.13 and 24, including real HTTP cancellation and timeout checks.
• Installed tarball imports pass in ESM and CommonJS, with strict TypeScript checks for both.
• Repository build, tests, lint, formatting, and type checks pass. The 14 existing credentialed integration tests were skipped.
• Live authenticated Search and Extract checks pass. The agent flow passes with a scripted model; a live model-provider run was not performed because RESEARCH_MODEL was unset.

This is an unpublished package proposal. No npm release is included. First publication still needs the npm owner bootstrap described in PUBLISHING.md; adding the package does not trigger a release.