.NET: Rename 3rdPartyThreadStorage sample to 3rdPartyChatHistoryStorage

[westey-m]

Motivation and Context

#3431

Description

• Rename 3rdPartyThreadStorage sample to 3rdPartyChatHistoryStorage

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 renames the Agent_Step07_3rdPartyThreadStorage sample to Agent_Step07_3rdPartyChatHistoryStorage to align with the terminology change from "thread storage" to "chat history storage" as requested in issue #3431.

Changes:

• Renamed the sample directory from Agent_Step07_3rdPartyThreadStorage to Agent_Step07_3rdPartyChatHistoryStorage
• Updated all references to the sample in documentation and solution files
• Enhanced code comments to better explain the sample's purpose and functionality

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
dotnet/samples/GettingStarted/Agents/README.md	Updated table entry to reference the renamed sample with new terminology
dotnet/samples/GettingStarted/Agents/Agent_Step07_3rdPartyChatHistoryStorage/Program.cs	Enhanced header comments to better explain the custom ChatHistoryProvider and session state persistence
dotnet/samples/GettingStarted/Agents/Agent_Step07_3rdPartyChatHistoryStorage/Agent_Step07_3rdPartyChatHistoryStorage.csproj	Project file for the renamed sample (consistent with other samples)
dotnet/agent-framework-dotnet.slnx	Updated solution reference to point to the renamed project