.NET Compaction - Allow developer to specify a custom formatter for ToolResultCompactionStrategy

[Copilot]

Motivation and Context

ToolResultCompactionStrategy used a hardcoded private formatter to collapse tool call groups into a YAML-like summary. There was no way to customize the output format, limiting extensibility for scenarios requiring different summary styles.

Description

• New toolCallFormatter constructor parameter — optional Func<CompactionMessageGroup, string>?; defaults to null (uses built-in format)
• ToolCallFormatter public property — exposes the active formatter so callers can inspect what's configured
• DefaultToolCallFormatter public static method — the former private BuildToolCallSummary, now public so custom formatters can delegate to or wrap the default output
• CompactCoreAsync updated — calls this.ToolCallFormatter(group) instead of the static method directly

// Use a custom formatter
var strategy = new ToolResultCompactionStrategy(
    trigger: CompactionTriggers.TokensExceed(8000),
    toolCallFormatter: group => $"[Tools used: {string.Join(", ", group.Messages.Select(m => m.Role))}]");

// Wrap the default formatter with additional context
var strategy = new ToolResultCompactionStrategy(
    trigger: CompactionTriggers.TokensExceed(8000),
    toolCallFormatter: group =>
        $"[Turn {group.TurnIndex}]\n{ToolResultCompactionStrategy.DefaultToolCallFormatter(group)}");

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? No — toolCallFormatter is an optional parameter with a null default; existing callers are unaffected.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>.NET Compaction - Allow developer to specify a custom formatter for ToolResultCompactionStrategy</issue_title>
<issue_description>We currently default to only one mode of compacting tool results. This might benefit by being extensible/injectable while retaining the existing approach as the default.</issue_description>

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

• Fixes .NET Compaction - Allow developer to specify a custom formatter for ToolResultCompactionStrategy #4600

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.