`code --status` multiplies process memory twice on macOS and Linux

Does this issue occur when all extensions are disabled?: Yes (the calculation is independent of extensions)

- VS Code Version: 1.133.0 (a5b500951314efd502d07465bd138dfbd714a960)
- OS Version: macOS 26.5.1, arm64

## Steps to reproduce

1. Run `code --status`.
2. Inspect the `Mem MB` values in the process table.

## Actual result

The command reports impossible process-memory values. In one current sample:

- `code --status`: 25,332,747,904 MB for PID 68258
- `ps` RSS for the same PID: 96,624 KiB (about 94 MiB)

## Root cause

`listProcesses` already normalizes `ProcessItem.mem` to bytes on non-Windows platforms:

```ts
mem: isWindows ? mem : (totalMemory * (mem / 100))
```

However, `DiagnosticsService.formatProcessItem`, which formats `code --status`, still treats `item.mem` as a percentage on macOS and Linux and multiplies it by total system memory again:

```ts
const memory = process.platform === 'win32' ? item.mem : (osLib.totalmem() * (item.mem / 100));
```

The formatter should consistently convert the byte value with `item.mem / ByteSize.MB`.

## Scope

This is adjacent to, but separate from, #194322 and #331139. Those address which memory metric Process Explorer obtains on macOS. This issue concerns the deterministic double conversion in the `code --status` text formatter. PR #331139 continues to provide `ProcessItem.mem` in bytes, so it does not correct this output path.

The same formatter branch is used on Linux, although the reproduction above is from macOS.