Update Azure Arc Connectivity Check drop (ArcEndpointCheck.ps1 v2.6.0)

[fabiotreze]

Description

This PR updates the Azure Arc Connectivity Check drop (script_automation/arc_endpoint_check) with a hardened, docs-aligned version of ArcEndpointCheck.ps1 (v2.6.0) and a refreshed _index.md.

What's changed

• Endpoint alignment with the official Connected Machine agent network requirements (core vs. optional groups: SQL, AMA, MDE, WAC).
• Automatic mode detection (Public vs. Azure Private Link) via azcmagent show -j and DNS/RFC1918 heuristic.
• Proxy awareness mirroring the agent precedence (-ProxyUrl > azcmagent proxy.url > HTTPS_PROXY); the Windows system-wide proxy is only reported, never applied, matching the agent. Honors azcmagent proxy.bypass categories to skip HTTP probes that the agent would not send through the proxy.
• IPv4 (A record) preference to avoid misclassifying Private Link endpoints that also expose a public AAAA record.
• Resilient GNS handling: dynamic allowlist (guestnotificationservice) failures are treated as WARN (not FAIL) so a transient SERVFAIL doesn't fail the run; DNS gets one retry.
• AMA metrics endpoint (global.prod.microsoftmetrics.com) added.
• Runs azcmagent check with the correct flags per mode (--enable-pls-check for Private; --extensions all --include-all with -CheckIncludeAll).
• Exit code 0 when all checks pass, 1 on any failure — usable in onboarding pipelines.

Files

• script_automation/arc_endpoint_check/ArcEndpointCheck.ps1
• script_automation/arc_endpoint_check/_index.md

Validated on a real Windows Server (Azure Arc-enabled, Private Link + Explicit Proxy) across all parameter combinations.