Fix expected conversation length in SummarizingChatReducer integration tests

[MackinnonBuck]

Overview

There were some recent changes made to SummarizingChatReducer to avoid orphaning an assistant response from the user message, so long as the unsummarized message count remains within the configured limit. The functional tests were updated, but the integration tests weren't, causing SummarizingChatReducer_PreservesConversationContext to fail with:

Message: 
Assert.Equal() Failure: Values differ
Expected: 3
Actual:   4

  Stack Trace: 
ChatClientIntegrationTests.SummarizingChatReducer_PreservesConversationContext() line 1160
--- End of stack trace from previous location ---

Changes

This PR updates the expected message count to 4 (summary + user message + assistant response + user message).

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR addresses a test failure in the integration tests for SummarizingChatReducer following recent changes (PR #6908) that modified the reducer's behavior to avoid orphaning assistant responses from user messages when within the configured threshold limit.

Key Changes:

• Updates the expected message count from 3 to 4 in the SummarizingChatReducer_PreservesConversationContext test to reflect the new behavior where the reducer searches backwards for a User message within the threshold window

Critical Issue Identified:
The PR updates the count assertion but does not update the subsequent Assert.Collection call, which still only validates 3 messages instead of the expected 4. This will cause the test to fail with an "Expected 4 elements but got 3" error.