.NET: Add context window size compaction strategy for harness

[westey-m]

Motivation and Context

Adding an opinionated compaction strategy that is easy to use based on context window size.

Description

• Add context window size compaction strategy for harness

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

Adds an opinionated “context window size”-driven compaction strategy to the .NET Agent Framework and wires it into the Harness research sample to keep long-running sessions within token budgets.

Changes:

• Introduces ContextWindowCompactionStrategy, which derives tool-eviction and truncation triggers from (maxContextWindowTokens - maxOutputTokens).
• Adds unit tests covering constructor validation and basic compaction behavior.
• Updates the Harness Step01 Research sample to use the new compaction strategy and display token usage breakdown during streaming.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI/Compaction/ContextWindowCompactionStrategy.cs	New strategy wrapping a compaction pipeline with thresholds computed from context window and output limits.
dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/ContextWindowCompactionStrategyTests.cs	New unit tests for constructor validation and compacting behavior.
dotnet/samples/02-agents/Harness/Harness_Step01_Research/Program.cs	Uses the new strategy in the Harness sample; switches to Foundry endpoint env var and configures local history reduction.
dotnet/samples/02-agents/Harness/Harness_Step01_Research/Harness_Step01_Research.csproj	Adds a project reference needed for OpenAI Responses extensions used by the sample.
dotnet/samples/02-agents/Harness/Harness_Shared_Console/HarnessConsole.cs	Enhances streaming console output to show token usage/reasoning and adds optional context-window parameters.

[westey-m]

Replying to open review comments with fixes applied — see individual thread replies below.