.NET: [BREAKING] Rename GetNewSession to CreateSession

[westey-m]

Motivation and Context

Since session is less tied to any specific underlying chat history storage mechanism, it is less likely to be confused for creating a service side artifact, so renaming to CreateSession.

#2542

Description

• Rename GetNewSession to CreateSession

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

This PR implements a breaking API change that renames GetNewSessionAsync to CreateSessionAsync across the entire Agent Framework codebase. The motivation is that "CreateSession" better reflects the operation since sessions are less tied to specific underlying chat history storage mechanisms, reducing potential confusion about whether service-side artifacts are being created.

Changes:

• Renamed the abstract method AIAgent.GetNewSessionAsync to AIAgent.CreateSessionAsync
• Updated all concrete implementations across 11+ agent types (ChatClientAgent, DurableAIAgent, WorkflowHostAgent, A2AAgent, CopilotStudioAgent, GitHubCopilotAgent, PurviewAgent, etc.)
• Updated all 115+ test files to use the new method name
• Updated 50+ sample applications and demonstration code
• Updated XML documentation comments, error messages, and README files to reference the new method name

Reviewed changes

Copilot reviewed 124 out of 124 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI.Abstractions/AIAgent.cs	Core abstract method renamed with comprehensive XML documentation
dotnet/src/Microsoft.Agents.AI.Abstractions/AgentSession.cs	XML documentation references updated to new method name
dotnet/src/Microsoft.Agents.AI.Abstractions/DelegatingAIAgent.cs	Delegation to inner agent updated
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgent.cs	Core implementation and 3 overloads renamed consistently
dotnet/src/Microsoft.Agents.AI.DurableTask/DurableAIAgent.cs	DurableTask implementation updated with error message fix
dotnet/src/Microsoft.Agents.AI.DurableTask/DurableAIAgentProxy.cs	Proxy implementation updated with error message fix
dotnet/src/Microsoft.Agents.AI.DurableTask/AgentEntity.cs	Entity implementation updated
dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowHostAgent.cs	Workflow host implementation updated
dotnet/src/Microsoft.Agents.AI.Workflows/Specialized/AIAgentHostExecutor.cs	Internal workflow executor updated
dotnet/src/Microsoft.Agents.AI.A2A/A2AAgent.cs	A2A agent and overload updated
dotnet/src/Microsoft.Agents.AI.CopilotStudio/CopilotStudioAgent.cs	Copilot Studio agent and overload updated
dotnet/src/Microsoft.Agents.AI.GitHub.Copilot/GitHubCopilotAgent.cs	GitHub Copilot agent and overload updated
dotnet/src/Microsoft.Agents.AI.Purview/PurviewAgent.cs	Purview wrapper agent updated
dotnet/src/Microsoft.Agents.AI.Hosting/NoopAgentSessionStore.cs	Session store implementation updated
dotnet/src/Microsoft.Agents.AI.Hosting/Local/InMemoryAgentSessionStore.cs	In-memory session store updated
dotnet/src/Microsoft.Agents.AI.Hosting.AzureFunctions/README.md	Azure Functions documentation updated
dotnet/tests/** (40+ files)	All test implementations and test method names updated
dotnet/samples/** (60+ files)	All sample code and documentation updated