.NET: Create a sample to show bounded chat history with overflow into chat history memory

[westey-m]

Motivation and Context

#3955

Description

• Create a sample that combines InMemoryChatHistoryStore with ChatHistoryMemoryProvider to keep a configurable number of messages in the session state, and offload any further messages into a vector store. On Run, the in-session messages will be augmented with a vector store search.

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 introduces a new sample (AgentWithMemory_Step05_BoundedChatHistory) that demonstrates how to combine InMemoryChatHistoryProvider with ChatHistoryMemoryProvider to implement a bounded chat history with vector store overflow. The sample keeps a configurable number of recent messages in session state and automatically archives older messages to a vector store, where they can be recalled via semantic search.

Changes:

• Adds a new sample showing bounded chat history with automatic overflow to vector store
• Implements TruncatingChatReducer to truncate chat history and expose removed messages
• Implements BoundedChatHistoryProvider to coordinate between in-memory and vector-based storage
• Updates the AgentWithMemory README to include the new sample

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
dotnet/samples/GettingStarted/AgentWithMemory/README.md	Adds entry for the new Step05 sample in the table of contents
dotnet/samples/GettingStarted/AgentWithMemory/AgentWithMemory_Step05_BoundedChatHistory/TruncatingChatReducer.cs	Implements IChatReducer to keep N most recent messages and expose removed messages
dotnet/samples/GettingStarted/AgentWithMemory/AgentWithMemory_Step05_BoundedChatHistory/README.md	Documents the sample's purpose, prerequisites, configuration, and how it works
dotnet/samples/GettingStarted/AgentWithMemory/AgentWithMemory_Step05_BoundedChatHistory/Program.cs	Sample demonstrating bounded chat history with 4-message session window and vector overflow
dotnet/samples/GettingStarted/AgentWithMemory/AgentWithMemory_Step05_BoundedChatHistory/BoundedChatHistoryProvider.cs	Custom ChatHistoryProvider coordinating InMemoryChatHistoryProvider and ChatHistoryMemoryProvider
dotnet/samples/GettingStarted/AgentWithMemory/AgentWithMemory_Step05_BoundedChatHistory/AgentWithMemory_Step05_BoundedChatHistory.csproj	Project file with required package references (Azure.AI.OpenAI, Azure.Identity, SemanticKernel.Connectors.InMemory)
dotnet/agent-framework-dotnet.slnx	Adds the new sample project to the solution