fix: honor proxyUser/proxyPass when downloading the Local binary

[Ashfaqbs]

Fixes #164.

Root cause

proxyHost/proxyPort are threaded through to every binary-download request path (the async in-process download, and the two spawned child scripts used by the sync path), but proxyUser/proxyPass were only ever forwarded to the already-running BrowserStackLocal binary's own --proxy-user/--proxy-pass flags (getBinaryArgs()). Local.js's conf object built in getBinaryPath() never carried them, so an authenticating proxy accepted the binary's own runtime traffic but rejected the request to download the binary in the first place.

Fix

Threaded proxyUser/proxyPass through the same four call sites that already handle proxyHost/proxyPort:

• LocalBinary.download() (async, in-process) and fetchDownloadSourceUrlAsync() — build the HttpsProxyAgent with an auth: "user:pass" option (what https-proxy-agent uses to set Proxy-Authorization).
• LocalBinary.downloadSync() and getSourceUrlSync(), which spawn download.js/fetchDownloadSourceUrl.js — pass the credentials through the child's environment (BROWSERSTACK_LOCAL_PROXY_USER/_PASS), not argv, mirroring this codebase's existing BROWSERSTACK_LOCAL_AUTH_TOKEN pattern (argv is readable via ps//proc/<pid>/cmdline, env is not).

Test plan

Added 5 regression tests in test/local.js (Proxy authentication for binary download), all offline/no network:

• the async download path builds the proxy agent with the right auth, and leaves it unset when no credentials are configured;
• both spawned-child paths pass credentials via env and never put them in argv;
• Local's own config building forwards proxyUser/proxyPass all the way through.

Confirmed each of the 5 fails against the pre-fix code and passes with the fix (stashed the lib/ changes, reran, restored). eslint lib/* index.js (the repo's own pretest) is clean.

I could not run the pre-existing "should download binaries with proxy" test (and the other real-network Download/Local tests) in this environment — they call the live local.browserstack.com endpoint and need a real BROWSERSTACK_ACCESS_KEY, neither of which I have here. Happy to have CI confirm those.

[coderabbitai]

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