fix(core): retry Chromium download with idle timeout (PER-9192)

[pranavz28]

Fixes PER-9192

Problem

Customers' CI builds intermittently fail with the Chromium download frozen near the end (e.g. Downloading Chromium 1300309 [=========== ] 167.3MB/178.1MB 93%). The build is created but uploads zero snapshots and is later reaped as manually_failed_build. It works locally because Chromium is already cached there; each CI runner starts cold and re-downloads the full ~178 MB, exposing it to network flakiness.

The Chromium artifact itself is fine on Google's bucket (HTTP 200, correct content-length, Range/resume supported) — the failure is a transient interruption of the connection that the CLI does not recover from.

Root cause

packages/core/src/install.js — download() performed a single https.get piped straight to a file:

• no socket/idle timeout → if data stops arriving but the socket isn't reset, the promise hangs until the whole CI job times out (the "stuck at 93%" symptom),
• no retry → one blip is fatal, even though an immediate retry almost always succeeds,
• no resume → a rerun re-downloads from scratch.

Fix

• Extract the per-attempt download into fetchToFile() with an idle socket timeout (request.setTimeout → request.destroy) so a stalled connection fails fast instead of hanging.
• Wrap it in a retry loop that removes the partial archive between attempts.
• Both tunable via PERCY_CHROMIUM_DOWNLOAD_TIMEOUT (default 30000 ms) and PERCY_CHROMIUM_DOWNLOAD_RETRIES (default 3).

This converts an indefinite hang into bounded, self-healing attempts. Behaviour is unchanged on the happy path and for permanent failures (e.g. a 404 still ultimately rejects, after exhausting retries).

Testing

• packages/client/test/helpers.js: taught the shared MockRequest helper to model setTimeout/destroy.
• packages/core/test/unit/install.test.js: added retries the download when an attempt fails and gives up after exhausting download retries.
• The three specs exercising the changed download() path (handles failed downloads, the two new ones) pass.

Note for reviewers

A momentary network glitch in CI currently kills the whole build. After this change it retries and recovers. The recommended customer-side mitigation (pre-installing a browser + PERCY_BROWSER_EXECUTABLE) still applies for fully air-gapped runners; this just makes the default path resilient.

🤖 Generated with Claude Code