Allow checking DNS with api.gihub.com.

[TingluoHuang]

https://github.com/github/actions-runtime/issues/5556

[Copilot]

Pull request overview

This PR extends the runner’s job-level connectivity diagnostics by adding an optional “connectivity + DNS” check path, intended to capture DNS resolution timing/IPs in addition to HTTP reachability, and emit richer telemetry to help diagnose endpoint resolution issues.

Changes:

• Adds a new ConnectivityAndDNSChecks payload path to schedule background checks that include DNS resolution.
• Enhances CheckConnectivity to optionally resolve DNS, capture resolved IPs, and split timing into DNS vs HTTP request duration.
• Adjusts emitted connectivity logs/telemetry formatting (JSON logging, new timing fields, status code prefixing).

Show a summary per file

File	Description
src/Runner.Worker/JobExtension.cs	Adds background connectivity+DNS checks and extends connectivity check result/telemetry with DNS/IP/timing data.

Review details

Comments suppressed due to low confidence (1)

src/Runner.Worker/JobExtension.cs:1212

• CheckServiceConnectivityAsync still treats only "OK" and "canceled" as non-failures, but CheckConnectivity now prefixes HTTP results (e.g. http_OK, http_canceled). This will incorrectly mark successful connectivity checks as failures and may cause misleading telemetry.

                        if (!testResult.HasFailure &&
                            result.StatusCode != "OK" &&
                            result.StatusCode != "canceled")
                        {

• Files reviewed: 1/1 changed files
• Comments generated: 5
• Review effort level: Low