.NET: Fix message ordering inconsistency when using AIContextProvider

[Copilot]

Motivation and Context

This PR fixes a message ordering inconsistency when using AIContextProvider in ChatClientAgent. The original issue reported that messages sent to the chat client were in a different order than messages stored in the MessageStore, causing confusion and potential issues with message context.

• Fixes .NET: Inconsistency of message ordering when using AIContextProvider #2655

Description

Fixed the message ordering to ensure consistency between messages sent to the chat client and messages stored in the MessageStore.

Before the fix:

• Sent to chat client: [Existing, AIContextProvider, Input]
• Stored in MessageStore: [Input, AIContextProvider, Response]

After the fix:

• Sent to chat client: [Existing, Input, AIContextProvider]
• Stored in MessageStore: [Input, AIContextProvider, Response]

Both now follow the same relative order where Input messages come before AIContextProvider messages.

// Example: Message flow with AIContextProvider
MessageStore: [A, B]                    // Existing
Input: [C]                              // User message
AIContextProvider returns: [X]          // Context injection

Sent to chat client: [A, B, C, X]       // Consistent order ✓
Stored after response: [C, X, D]        
Final MessageStore: [A, B, C, X, D]     // Consistent order ✓

Changes Made:

• ChatClientAgent.cs: Reordered how messages are added in PrepareThreadAndMessagesAsync method to add input messages before AIContextProvider messages
• ChatClientAgentTests.cs: Updated existing tests that validate the order of messages sent to the chat client

The fix ensures that:

1. Messages sent to chat client follow order: Existing, Input, AIContextProvider
2. New messages are stored in order: Input, AIContextProvider, Response
3. Both maintain consistent relative ordering of Input before AIContextProvider messages

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.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>.NET: Inconsistency of message ordering when using AIContextProvider</issue_title>
<issue_description>Maybe I am mistaken and there is something that I am missing, but currently it appears when using an AIContextProvider, the message order is different when sending it to the chat client vs when sending it to the message store.

Let's say, we have two messages in the message store already:

Message Store:

1. Message A
2. Message B

Then we invoke the agent another time with Message C.

Let's assume the ContextProvider injects Message X.

The following messages are sent to the chat client (see PrepareThreadAndMessagesAsync method):

1. Message A
2. Message B
3. Message X
4. Message C

The chat client returns Message D.

However, the messages are then stored in the following order in the message store (see invokation of NotifyMessageStoreOfNewMessagesAsync

1. Message C
2. Message X
3. Message D</issue_description>

<agent_instructions>Ensure that the order is always:

1. Existing Messages from the store
2. Messages from the AIContextProvider
3. New Messages from the User
4. Response from the agent.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes .NET: Inconsistency of message ordering when using AIContextProvider #2655

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