.NET: Prevent streamed updates loss when resuming streaming

[SergeyMenshykh]

This PR disables scenarios that would experience message loss, such as streaming resumption with threads configured with either a custom message store or a context provider.

Closes: #1760

[Copilot]

Pull request overview

This PR addresses a critical issue with message loss during streaming resumption by adding validation to prevent unsupported scenarios. The changes ensure that continuation tokens cannot be used with custom message stores or context providers during streaming operations, where collected updates could be lost.

Key changes:

• Added validation in ValidateStreamResumptionAllowed to prevent streaming resumption with MessageStore or AIContextProvider
• Added validation to prevent continuation tokens from being used on initial runs (when thread has no ConversationId and no MessageStore)
• Updated existing tests to include ConversationId in thread and response objects to ensure tests represent realistic continuation scenarios
• Added comprehensive test coverage for all new validation scenarios

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgent.cs	Implements new validation logic to prevent streaming resumption with MessageStore or AIContextProvider, and prevents continuation tokens on initial runs
dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgentTests.cs	Updates existing tests to use ConversationId and adds four new test cases covering all validation scenarios for continuation token usage