feat(security): unblock public CA cluster deployments

[yordis]

• Managed certificate authorities may omit the Client Authentication usage, preventing secure nodes from authenticating during cluster operations.
• Strict validation should remain the default so operators make this compatibility decision explicitly.
• Kubernetes deployments need a supported path for externally managed node certificates without weakening trust or node identity checks.

[cursor]

PR Summary

Medium Risk
Changes cluster node identity checks for HTTPS when operators opt in; default remains strict, but misconfiguration could weaken mutual TLS expectations for inter-node HTTP.

Overview
Adds explicit compatibility for public-CA node certificates that include Server Authentication but not Client Authentication in the Extended Key Usage extension—common when the same cert is used for node-to-node HTTPS.

New configuration AllowNodeCertificateWithoutClientAuthEku (CLI, YAML, EVENTSTORE_ALLOW_NODE_CERTIFICATE_WITHOUT_CLIENT_AUTH_EKU) defaults to false. When enabled, IsServerCertificate can treat server-auth-only certs as valid node server certs; NodeCertificateAuthenticationProvider applies the policy for incoming HTTPS cluster connections. Reserved node CN, SAN, chain, and Server Authentication requirements are unchanged, and user certificate authentication is not relaxed. The flag is read via a reloadable delegate from ClusterVNode (including SIGHUP config reload).

Documentation in docs/security.md describes when to enable this and notes it does not affect legacy secure TCP replication certificate handling.

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Review details

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Configuration used: Organization UI

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Run ID: 4c3ceabe-84c8-4e0c-9069-2d6bbf0035e1

📥 Commits

Reviewing files that changed from the base of the PR and between d9af4ae and ded7510.

📒 Files selected for processing (8)

• docs/security.md
• src/EventStore.Common/Utils/CertificateExtensions.cs
• src/EventStore.Core.Tests/Certificates/key_usages.cs
• src/EventStore.Core.Tests/Services/Transport/Http/Authentication/node_certificate_authentication_provider.cs
• src/EventStore.Core.XUnit.Tests/Configuration/ClusterVNodeOptionsTests.cs
• src/EventStore.Core/ClusterVNode.cs
• src/EventStore.Core/Configuration/ClusterVNodeOptions.cs
• src/EventStore.Core/Services/Transport/Http/Authentication/NodeCertificateAuthenticationProvider.cs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch yordis/feat-certificate-eku-policy

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