.NET: [Bug]: Token Count Discrepancy

### Description

Token counts passed to compaction strategies through the `index` parameter of `CompactCoreAsync` function and `UsageContent` object the `RunStreamingAsync` returns when an agent is called differs even though the compaction is not engaged at all. The token count reported by compaction strategy is less than the count reported in `UsageContent`.

I am comparing `CompactionMessageIndex.IncludedTokenCount` and `UsageContent.Details.InputTokenCount`

The difference usually big enough so it can't be ignored (e.g. Compaction reports: 2435 input tokens vs 12202 input tokens by UsageContent)

### Code Sample

```markdown

```

### Error Messages / Stack Traces

```markdown

```

### Package Versions

Microsoft.Agents.AI 1.4.0

### .NET Version

.NET10

### Additional Context

_No response_