Fix false offline detection and hanging web requests

[PtJade-Ceramic]

Summary

Fixes #2782: on machines where the c-ares DNS resolver fails but the system is online (e.g. VPN / proxy / hosts-file setups, common in some regions), the extension misreports the machine as offline ("It looks like you may be offline...") and installs fail after the full timeout with DotnetOfflineFailure.

The actual network was reachable (verified: dns.lookup resolves, HTTPS to builds.dotnet.microsoft.com returns 200) — the bug is in the offline-detection and request-timeout code, not the environment.

Changes

vscode-dotnet-runtime-library/src/Utils/WebRequestWorkerSingleton.ts

1. isOnline() no longer uses a c-ares Resolver

• Replaced dns.promises.Resolver (c-ares, reads a static DNS server list) with dns.promises.lookup, which uses the same resolution path as real socket connections (hosts file, VPN adapters, system DNS).
• The DNS timeout budget is corrected from timeoutSec * 2 ms (1.2s at the default 600s — 10× shorter than the original comment's "1/50th of the download time") to Math.max(timeoutSec * 20, 5000) (12s at default, 5s floor).
• DNS errors/timeouts still post OfflineDetectionLogicTriggered and return false, so genuinely-offline behavior is unchanged.
• Added a protected resolveHostnameForOnlineCheck() so the resolver is testable.

2. Hung requests fail fast instead of blocking for the full timeout

• Non-download requests now use a bounded 30s per-attempt timeout (attemptTimeoutMs), so a connection that hangs — e.g. one routed through a broken/partial proxy — fails quickly and triggers the existing native fetch (no-proxy) fallback, instead of blocking for the full user-configured timeout (default 600s).
• Streaming downloads (installer downloads) keep the full user-configured budget so large files aren't cut off on slow connections.

3. getCachedData()/makeWebRequest() accept optional per-request axios options (backward-compatible) so callers can override e.g. the cache TTL. Used by the fixed test below.

Tests

• vscode-dotnet-runtime-library/src/test/unit/WebRequestWorker.test.ts:
  • Added 4 isOnline() unit tests (resolves / does not resolve / DNS error / DOTNET_INSTALL_TOOL_OFFLINE).
  • Fixed the pre-existing, systematically-failing Web Requests Cached Does Not Live Forever test: it waited exactly the configured 120s TTL, but axios-cache-interceptor interprets the server's cache headers (default interpretHeader: true), so the entry's effective TTL is ~10 minutes (e.g. microsoft.com/example.com return a long max-age) and the 120s wait never observed expiry — failing on every platform. The test now forces a 2s TTL with interpretHeader disabled, waits 3s, and asserts the entry expired: fast and deterministic.
  • Gave It actually times requests an explicit timeout so it is robust on slow networks.
• vscode-dotnet-runtime-library/src/test/mocks/MockObjects.ts: MockTrackingWebRequestWorker forwards the new optional request options.

Changelog

• [Unreleased] entries added to both the runtime and SDK extension CHANGELOGs.

Validation

• npm run compile passes.
• All WebRequestWorker unit tests pass locally (11/11), including the previously-failing cache-expiry test and the new isOnline() tests; LocalInstallUpdateService (11/11) also passes.
• Reproduced the root cause locally: c-ares Resolver → ECONNREFUSED for www.microsoft.com, while dns.lookup resolves in ~26ms and HTTPS to builds.dotnet.microsoft.com returns HTTP 200.

Notes

• No version bump (per repo convention).
• Follow-up (out of scope): the deprecated get-proxy-settings dependency could be replaced with a maintained alternative, and installer downloads could fail over to alternate Microsoft CDNs.

[PtJade-Ceramic]

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[PtJade-Ceramic]

About the failing macOS check

The macOS Build and Test failure is a pre-existing, repo-wide CI infrastructure issue — it is not caused by this PR.

Evidence:

• All library unit tests pass on all three platforms (280 passing, 0 failing on macOS/linux/windows), including the previously-flaky cache-expiry test that this PR fixes.
• linux and windows Build and Test pass fully (including their extension functional tests).
• macOS fails only in the extension functional tests, with:

Test error: Error: spawn /Users/runner/work/1/s/vscode-dotnet-runtime-extension/.vscode-test/vscode-darwin-1.135.0/Visual Studio Code.app/Contents/MacOS/Electron ENOENT

This is @vscode/test-electron failing to launch the downloaded VS Code on the macOS agent — it happens before any of this PR's code executes.

• The same macOS failure affects the unrelated open PR Create SECURITY.md for security policy #2769 (Create SECURITY.md), whose CI status matches this PR exactly (macOS fail, linux/windows pass).

So this PR's changes are green on the code itself; the macOS job is red due to the shared CI infrastructure. Happy to rebase/fix if the underlying macOS test-launch issue is addressed.