[watch aspire] Suppress OperationCanceledException in StartChannelReader on shutdown

[Copilot]

On Ctrl+C, dotnet watch was logging a spurious ❌ Unexpected error reading output of session due to a TaskCanceledException that should have been silently ignored.

Root cause

StartChannelReader receives runningProject.Process.ExitedCancellationToken (cancelled only when the child process exits). It guarded against false-positive error logs by checking !cancellationToken.IsCancellationRequested. However, SendMessageAsync internally builds a linked token from the passed token plus _shutdownCancellationSource.Token, so pressing Ctrl+C cancels the send while the process exit token is still live — causing the guard to miss the cancellation and log the error.

Fix

Replace the single catch + conditional log with two separate catches, matching the existing pattern used in the onExit callback in the same method:

// before
catch (Exception e)
{
    if (!cancellationToken.IsCancellationRequested)
    {
        _logger.LogError("Unexpected error reading output of session '{SessionId}': {Exception}", sessionId, e);
    }
}

// after
catch (OperationCanceledException)
{
    // canceled on shutdown, ignore
}
catch (Exception e)
{
    _logger.LogError("Unexpected error reading output of session '{SessionId}': {Exception}", sessionId, e);
}

[Copilot]

Pull request overview

This PR adjusts dotnet watch Aspire session output handling to avoid logging a misleading “Unexpected error reading output of session …” message during shutdown (e.g., Ctrl+C), by explicitly suppressing cancellation exceptions from the channel reader loop.

Changes:

• Handle OperationCanceledException separately in StartChannelReader so cancellation during shutdown doesn’t produce an error log.
• Simplify the remaining exception path to always log genuinely unexpected failures.