[dotnet-code] Align compaction equality helpers

[Michelle Clayton (michelle-clayton-work)]

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The original pull request description is below.

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Summary

Extracts unexported per-content comparison helpers from agent/compaction/equality.go. This keeps the existing compaction equality behavior while making the Go implementation structurally closer to .NET's ChatMessageContentEquality helper layout for future ports.

.NET Reference

• dotnet/src/Microsoft.Agents.AI/Compaction/ChatMessageContentEquality.cs - defines per-content comparison helpers used by compaction message matching.

Public API and Behavior

No public Go API changed. No intentional behavior change was made.

Tests

• go test ./agent/compaction

Notes

Rejected candidates:

• dotnet/src/Microsoft.Agents.AI.Abstractions/AgentResponse.cs / AgentResponseUpdate.cs - comparable Go response assembly already had aligned behavior; changing update metadata handling would risk observable behavior changes.
• dotnet/src/Microsoft.Agents.AI.Workflows/MessageMerger.cs - comparable Go workflow merging internals were close, but usage/raw representation parity questions were behavior-affecting rather than safe cleanup.
• dotnet/src/Microsoft.Agents.AI/Skills/* - inspected as a sampled area from the upstream directory listing, but no safer small cleanup was found before the compaction equality candidate.

The direct upstream git fetch was blocked by the environment, so the .NET reference was inspected through the read-only GitHub bridge after checking local refs.

Closes #1080

[Copilot]

🟢 Approval recommended

No unresolved issues were identified, and behavior remains unchanged.

Pull request overview

Refactors compaction content equality into focused unexported helpers while preserving behavior and API compatibility.

Changes:

• Extracted per-content equality helpers.
• Retained existing comparison logic and recursive behavior.

File summaries

File	Description
agent/compaction/equality.go	Adds per-content equality helpers used by compaction matching.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Lite

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[github-actions]

Scope: internal-only

Changed Go contract: None. agent/compaction/equality.go extracts the existing inline per-content comparison logic from contentEqual into new unexported helper functions (textContentEqual, textReasoningContentEqual, dataContentEqual, uriContentEqual, errorContentEqual, functionCallContentEqual, functionResultContentEqual, hostedFileContentEqual). All extracted logic is byte-for-byte identical to the code it replaces; no exported identifiers, signatures, or comparison semantics changed.

Upstream evidence reviewed: dotnet/src/Microsoft.Agents.AI/Compaction/ChatMessageContentEquality.cs (cited in the PR description) uses per-content-type static comparison helpers analogous to the ones now extracted in Go. No behavior-affecting divergence was introduced by this restructuring, so no further upstream comparison was needed.

Result: aligned — this is a pure internal refactor (unexported helper extraction) with no observable behavior change and no public API surface change. No public-api-change label is warranted, and none is currently present. No parity-approved/public-api-change label cleanup was needed since neither is present on the PR.

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