[Server] Add several interfaces to improve testability & prepare for Dependency Injection

[romanett]

Proposed changes

Add IConfigurationNodeManager, IDiagnosticsNodeManager, IMasterNodeManager, IStandardServer, IApplicationInstance interfaces to Server. Add IMainNodeManagerFactory to create ConfigurationNodeManager & CoreNodeManager.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which adds functionality)
• Test enhancement (non-breaking change to increase test coverage)
• Breaking change (fix or feature that would cause existing functionality to not work as expected, requires version increase of Nuget packages)
• Documentation Update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING doc.
• I have signed the CLA.
• I ran tests locally with my changes, all passed.
• I fixed all failing tests in the CI pipelines.
• I fixed all introduced issues with CodeQL and LGTM.
• I have added tests that prove my fix is effective or that my feature works and increased code coverage.
• I have added necessary documentation (if appropriate).
• Any dependent changes have been merged and published in downstream modules.

[Copilot]

Pull request overview

This pull request adds several interfaces to improve testability and prepare for Dependency Injection in the OPC UA .NET Standard Server implementation. The changes introduce interface abstractions for key server components including StandardServer, MasterNodeManager, and various NodeManager types.

Changes:

• Added six new interfaces: IStandardServer, IMasterNodeManager, IMainNodeManagerFactory, ICoreNodeManager, IDiagnosticsNodeManager, and IConfigurationNodeManager
• Modified existing classes to implement these new interfaces
• Changed return types from concrete classes to interfaces in multiple methods
• Added IApplicationInstance interface for ApplicationInstance class
• Introduced MainNodeManagerFactory to support factory-based instantiation

Reviewed changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated 29 comments.

Show a summary per file

File	Description
Libraries/Opc.Ua.Server/Server/IStandardServer.cs	New interface defining the standard server contract
Libraries/Opc.Ua.Server/Server/StandardServer.cs	Implements IStandardServer, adds factory creation pattern for node managers
Libraries/Opc.Ua.Server/NodeManager/IMasterNodeManager.cs	New interface for master node manager with comprehensive API surface
Libraries/Opc.Ua.Server/NodeManager/MasterNodeManager.cs	Implements IMasterNodeManager, uses factory for creating core node managers
Libraries/Opc.Ua.Server/NodeManager/IMainNodeManagerFactory.cs	New factory interface for creating main node managers
Libraries/Opc.Ua.Server/NodeManager/MainNodeManagerFactory.cs	Factory implementation for creating ConfigurationNodeManager and CoreNodeManager
Libraries/Opc.Ua.Server/NodeManager/ICoreNodeManager.cs	New interface for core node manager
Libraries/Opc.Ua.Server/NodeManager/CoreNodeManager.cs	Implements ICoreNodeManager, fixes lock usage, makes ImportNodes public
Libraries/Opc.Ua.Server/Diagnostics/IDiagnosticsNodeManager.cs	New interface for diagnostics node manager
Libraries/Opc.Ua.Server/Diagnostics/DiagnosticsNodeManager.cs	Implements IDiagnosticsNodeManager, changes method visibility
Libraries/Opc.Ua.Server/Configuration/IConfigurationNodeManager.cs	New interface for configuration node manager
Libraries/Opc.Ua.Server/Configuration/ConfigurationNodeManager.cs	Implements IConfigurationNodeManager
Libraries/Opc.Ua.Server/Server/IServerInternal.cs	Adds MainNodeManagerFactory and ConfigurationNodeManager properties, uses interface types
Libraries/Opc.Ua.Server/Server/ServerInternalData.cs	Uses interface types, adds MainNodeManagerFactory support
Libraries/Opc.Ua.Configuration/IApplicationInstance.cs	New interface for application instance
Libraries/Opc.Ua.Configuration/ApplicationInstance.cs	Implements IApplicationInstance, uses IServerBase
Tests/Opc.Ua.Client.Tests/ReferenceServerWithLimits.cs	Updates CreateMasterNodeManager to return interface type
Libraries/Opc.Ua.Gds.Server.Common/GlobalDiscoverySampleServer.cs	Updates CreateMasterNodeManager to return interface type
Applications/Quickstarts.Servers/Utils.cs	Updates method signatures to use IStandardServer
Applications/Quickstarts.Servers/ReferenceServer/ReferenceServer.cs	Updates CreateMasterNodeManager to return interface type