fix(amazonq): stream large ATX artifact downloads to disk

[xueningl-az]

Problem

Ticket V2381716118 — a customer (United Airlines, VS2026) could not download a
large Transformation_Report.html from the Artifact Explorer. In the LSP
logs, CreateArtifactDownloadUrl succeeds every time, yet the file never
lands and no error surfaces.

Root cause is in downloadArtifactToPath:

const response = await got.get(url, { responseType: 'buffer', timeout: {
request: 30000 } })
fs.writeFileSync(filePath, Buffer.from(response.body))
- timeout: { request: 30000 } caps the entire request, including body
  transfer. A large report that takes >30s to download aborts with a
  TimeoutError.
- The error is swallowed by the catch and returned only as a generic { 
  Success: false }, so the user sees a silent failure.
- responseType: 'buffer' also buffers the whole file (plus a copy) in memory.

Fix

- Stream the presigned URL straight to disk with got.stream + stream/promises
  pipeline — no full-file in-memory buffering.
- Replace the whole-request timeout with inactivity-based timeouts (response
  30s to headers, socket 60s idle) so a large-but-healthy download is no
  longer capped by wall-clock time.
- Add success/error logging (bytes transferred, failure reason) for
  diagnosability.

Testing

- Added 3 unit tests for downloadArtifactToPath (large-file streaming success,
  mid-stream failure surfaces the error, existing URL-failure branch).
- npm run test:unit for aws-lsp-codewhisperer: 297 passing, 0 failing.
- ESLint/Prettier clean on the changed files.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 2.43902% with 40 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...anguage-server/netTransform/atxTransformHandler.ts	2.43%	40 Missing ⚠️

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