[TW-4918] Improve attachment download stream developer experience

[AaronDDM]

Summary

• add attachments.downloadNodeStream() as a non-breaking Node.js convenience helper that converts attachment downloads to NodeJS.ReadableStream
• keep attachments.download() returning Web ReadableStream for Fetch-native runtimes such as Cloudflare Workers
• use explicit node:stream imports for Node built-ins, consistent with the SDK's existing Cloudflare Workers requirement for nodejs_compat
• update attachments.download() JSDoc to describe the actual Web Stream return type
• add attachment resource tests for the Node stream helper
• add an Unreleased CHANGELOG entry

Jira

• TW-4918

Why

The v8 native Fetch migration changed Response.body to the Web Streams API. That is the right cross-runtime primitive, but Node.js production apps commonly need Node streams for pipeline(), file writes, S3 uploads, and HTTP response streaming. This PR preserves the existing API while adding an explicit Node-friendly path.

The SDK already requires Cloudflare Workers deployments to enable nodejs_compat, as shown by the existing Workers examples and top-level Node built-in usage in the SDK. Using node:stream is therefore explicit and consistent rather than introducing a new compatibility requirement.

Validation

• npm test -- attachments.spec.ts --coverage=false
• npm run build
• npm run lint -- --quiet

Note: npm test -- attachments.spec.ts passes the attachment suite, but exits nonzero when coverage is enabled because this repo enforces global coverage thresholds for focused test runs.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.72%. Comparing base (f005620) to head (8f739b0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #731   +/-   ##
=======================================
  Coverage   98.72%   98.72%           
=======================================
  Files          39       39           
  Lines         860      864    +4     
  Branches       69       69           
=======================================
+ Hits          849      853    +4     
  Misses         11       11           

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