[Repo Assist] refactor: extract ModelFormatting helper; deduplicate FormatTokenCount/FormatCount

[github-actions]

🤖 This is an automated draft PR from Repo Assist.

Summary

FormatTokenCount (in SessionInfo) and FormatCount (in GatewayUsageInfo) were byte-for-byte identical private static methods:

private static string FormatTokenCount(long n)  // SessionInfo
private static string FormatCount(long n)        // GatewayUsageInfo
{
    if (n >= 1_000_000) return $"{n / 1_000_000.0:F1}M";
    if (n >= 1_000) return $"{n / 1_000.0:F1}K";
    return n.ToString();
}
```

Both format a `long` with K/M suffix for compact display — a clear DRY violation that would require keeping two copies in sync if the format logic ever changed.

## Change

- Removed both duplicated private methods
- Added a new `internal static class ModelFormatting` (file-scoped, at the bottom of `Models.cs`) with a single `FormatLargeNumber(long n)` method
- Updated both callsites to use `ModelFormatting.FormatLargeNumber(...)`

No behaviour change — output is identical.

## Why this placement?

`ModelFormatting` lives at the bottom of `Models.cs` (file-scoped) so it stays co-located with the model types that use it, without introducing a new file for a 6-line helper. It's `internal` so it's available to the test project if anyone wants to add unit tests for the formatting logic.

## Test Status

503 tests pass, 18 skipped (infrastructure) — same as baseline.

```
Passed!  - Failed: 0, Passed: 503, Skipped: 18, Total: 521

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