Setting `LargeOutput` doesn't seem to have any effect

By default, sessions write large outputs to the` $TMP` directory (e.g. `/tmp/1782206406834-copilot-tool-output-s4fxvs.txt`). We tried to set `LargeOutput` to redirect the files to a different directory but it doesn't seem to be change the target directory:

```cs
config.LargeOutput = new LargeToolOutputConfig
{
Enabled = true,
MaxSizeBytes = 2_000,
OutputDirectory = outputDir,
}
```

Start by reproducing the C# configuration shown in the issue with a small MaxSizeBytes value and a custom OutputDirectory. Trace how LargeOutput is handled when tool output exceeds that limit and verify which directory receives the file. Done means oversized outputs are written to the configured directory instead of $TMP.

索引モデルが issue の本文から書いたものです。