Update Azure Arc Connectivity Check drop (ArcEndpointCheck.ps1)

[Fabio Rodrigues Vieira Costa (fabiotreze)]

Description

Follow-up to #1908 (merged). This PR brings post-merge refinements to the
Azure Arc Connectivity Check drop (script_automation/arc_endpoint_check):
a hardened ArcEndpointCheck.ps1 and a refreshed _index.md.

Capabilities in this revision

• Connectivity modes: Public, Private, Gateway, or Auto detection.
• Platform context: Arc, AzureLocal, AzureStackHub, or AzureVM.
• Core + extension endpoint validation (SQL, AMA, MDE, WAC, Key Vault, Hybrid Worker,
  Update Manager, Guest Attestation, Dependency Agent, Defender for SQL) with auto-detection.
• DNS/TCP/HTTP/TLS probing, including SCHANNEL TLS 1.2 posture and a live TLS 1.2 handshake.
• PKI / OCSP / CRL reachability and bypass-coverage validation.
• Proxy awareness (WinHTTP, azcmagent proxy settings, HTTPS_PROXY, bypass categories).
• Split-network handling for Private mode: an ARM/management.azure.com path failure is a
  non-blocking warning (not a runtime outage) when Arc private endpoints and Guest
  Configuration are healthy and azcmagent check reports critical_failures=0.
• Pre-onboarding mode when the Connected Machine agent is not installed (DNS/TCP/HTTP/TLS only).

Files

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

Validated on a real Windows Server (Azure Arc-enabled) across Public, Private Link, and
Gateway scenarios with explicit-proxy configurations.

[microsoft-github-policy-service]

👋 Hello Fabio Rodrigues Vieira Costa (@fabiotreze),

Thank you for your pull request, a team member will review it soon. All issues and pull requests are validated by the Arc Jumpstart team and must pass all validation checks before being merged.

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