[dotnet-code] Consolidate truncation removable count

[michelle-clayton-work]

Reuse the message index helper for counting included non-system groups before truncation and add a preservation test for skipped system and pre-excluded groups.

• [dotnet-code] Consolidate truncation removable count #379

[Copilot]

Pull request overview

This PR aligns Go truncation compaction internals with the existing “included non-system groups” concept (matching the .NET reference noted in #379) by reusing the shared message-index helper for removable-group counting, and adds a regression test to ensure truncation preserves system groups and respects pre-excluded groups.

Changes:

• Replaced truncation’s inline removable-group counting loop with MessageIndex.IncludedNonSystemGroupCount().
• Added TestTruncationStrategy_SkipsPreExcludedAndSystemGroups to validate behavior around system groups, pre-excluded groups, and the minimum-preserved floor.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
agent/compaction/truncation.go	Reuses the shared index helper to compute removable non-system group count, reducing duplicated logic.
agent/compaction/compaction_test.go	Adds a focused test ensuring truncation skips system and pre-excluded groups while still excluding the oldest eligible group.

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[github-actions]

Cross-Repo API Consistency Review

Result: ✅ No parity issues — internal refactor only.

Both changed files are out of scope for public-contract parity review:

• agent/compaction/truncation.go: Replaces a 5-line inline loop with a call to the existing index.IncludedNonSystemGroupCount() helper. The TruncationStrategy exported struct, its fields, and its Compact method signature are all unchanged. Observable behavior is identical.
• agent/compaction/compaction_test.go: Adds a test for the pre-excluded and system-group skip edge case. No API changes.

Upstream alignment note: The .NET CompactionMessageIndex already exposes the equivalent property (public int IncludedNonSystemGroupCount), so the underlying helper concept is shared. The .NET TruncationCompactionStrategy still uses the inline loop rather than calling that property, but that is an internal implementation detail — not a public-contract divergence.

No inline comments needed.

Generated by Go API Consistency Review Agent for #396 · 173.6 AIC · ⌖ 12.5 AIC · ⊞ 28.8K · ◷