[John Karam (johnkaram02)]

Motivation and Context

Fixes #14654.

ChatHistoryTruncationReducer and ChatHistorySummarizationReducer both accept targetCount: 1. When the history contains a system message, ReduceAsync throws ArgumentOutOfRangeException. LocateSafeReductionIndex subtracts the system message from the target count, which leaves 0, and then reads chatHistory[chatHistory.Count]. So a reducer configured this way fails as soon as the history exceeds the target.

Description

• ChatHistoryReducerExtensions.LocateSafeReductionIndex still counts the system message toward the target, but now always keeps at least one non-system message. With targetCount: 1, truncation keeps the system message and the latest message, and summarization keeps the system message, the summary, and the latest message.
• Behavior is unchanged for targetCount values greater than 1, and for histories without a system message.
• Added a unit test for each reducer that covers targetCount: 1 with a system message. Both tests fail on main with ArgumentOutOfRangeException and pass with this change.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

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[Copilot]

🟢 Approval recommended

The localized fix addresses the invalid index, with focused regression coverage for both reducers and no unresolved findings.

No open issues 🎉

What changed in this PR

Fixes #14654 by preventing chat history reducers from indexing past the history when targetCount: 1 and a system message is present.

Changes:

• Keeps at least one non-system message while preserving behavior for other valid target counts.
• Adds regression tests confirming truncation retains the system and latest messages, and summarization also retains a summary.

File	Description
dotnet/​src/​SemanticKernel.UnitTests/​AI/​ChatCompletion/​ChatHistoryTruncationReducerTests.cs	Tests truncation with a system message and target count of one.
dotnet/​src/​SemanticKernel.UnitTests/​AI/​ChatCompletion/​ChatHistorySummarizationReducerTests.cs	Tests summarization with a system message and target count of one.
dotnet/​src/​SemanticKernel.Core/​AI/​ChatCompletion/​ChatHistoryReducerExtensions.cs	Clamps the adjusted target count to at least one.

_{Review effort: Balanced}