.NET: Improve session cast error message quality and consistency

[westey (westey-m)]

Motivation and Context

#3677

Description

• Improve quality and consistency of session cast failures

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 improves the clarity and consistency of error messages when session type mismatches occur during agent operations. The changes address feedback from issue #3677 that the previous error messages were unclear about whether they referred to the agent instance or the agent type.

Changes:

• Updated all session type mismatch error messages to include both the actual session type name and the expected session type name
• Standardized error message format across all agent implementations (ChatClientAgent, CopilotStudioAgent, A2AAgent, GitHubCopilotAgent, DurableAIAgent, DurableAIAgentProxy, WorkflowHostAgent)
• Applied consistent error messages in test agent implementations

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgent.cs	Updated session type mismatch errors in SerializeSessionCoreAsync and RunCoreStreamingAsync to include actual and expected type names
dotnet/src/Microsoft.Agents.AI.CopilotStudio/CopilotStudioAgent.cs	Updated session type mismatch errors in SerializeSessionCoreAsync, RunCoreAsync, and RunCoreStreamingAsync
dotnet/src/Microsoft.Agents.AI.A2A/A2AAgent.cs	Updated session type mismatch errors in SerializeSessionCoreAsync and GetA2ASessionAsync
dotnet/src/Microsoft.Agents.AI.GitHub.Copilot/GitHubCopilotAgent.cs	Updated session type mismatch errors in SerializeSessionCoreAsync and RunCoreStreamingAsync to use consistent format
dotnet/src/Microsoft.Agents.AI.DurableTask/DurableAIAgent.cs	Updated session type mismatch error in SerializeSessionCoreAsync
dotnet/src/Microsoft.Agents.AI.DurableTask/DurableAIAgentProxy.cs	Updated session type mismatch error in SerializeSessionCoreAsync
dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowHostAgent.cs	Updated session type mismatch error in SerializeSessionCoreAsync
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/TestEchoAgent.cs	Updated session type mismatch error in test agent
dotnet/tests/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests/AGUIEndpointRouteBuilderExtensionsTests.cs	Updated session type mismatch errors in test agents
dotnet/tests/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.IntegrationTests/SharedStateTests.cs	Updated session type mismatch error in test agent
dotnet/tests/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.IntegrationTests/ForwardedPropertiesTests.cs	Updated session type mismatch error in test agent
dotnet/tests/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.IntegrationTests/BasicStreamingTests.cs	Updated session type mismatch error in test agent