Add TTY detection for interactive prompts to prevent agent hangs (#1547)

* Initial plan

* Add TTY detection for interactive prompts to prevent agent hangs

- CertCommand: detect non-interactive stdin before prompting, fail with
  clear error directing users to --force flag
- ProxyEngine: skip first-run cert trust prompt in non-interactive mode,
  default to trusting the certificate
- Update --force option help text to note it's required for non-interactive use

Co-authored-by: waldekmastykarz <11164679+waldekmastykarz@users.noreply.github.com>

* Address review feedback: add CI env var check and return non-zero exit code

- CertCommand and ProxyEngine: check both Console.IsInputRedirected and
  CI env var for non-interactive detection, matching ExecuteAsync pattern
- RemoveCert returns int: exit code 1 for non-interactive failure or
  exceptions, 0 for success

Co-authored-by: waldekmastykarz <11164679+waldekmastykarz@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: waldekmastykarz <11164679+waldekmastykarz@users.noreply.github.com>
Co-authored-by: Waldek Mastykarz <waldek@mastykarz.nl>
Co-authored-by: Garry Trinder <garry@trinder365.co.uk>

Author: 3 people

DevProxy/Commands/CertCommand.cs

@@ -15,7 +15,7 @@ sealed class CertCommand : Command
     private readonly ILoggerFactory _loggerFactory;
     private readonly Option<bool> _forceOption = new("--force", "-f")
     {
-        Description = "Don't prompt for confirmation when removing the certificate"
+        Description = "Don't prompt for confirmation when removing the certificate. Required for non-interactive use (CI, piped stdin, automation)."
     };
 
     public CertCommand(ILogger<CertCommand> logger, ILoggerFactory loggerFactory) :
@@ -74,7 +74,7 @@ private async Task EnsureCertAsync()
         _logger.LogTrace("EnsureCertAsync() finished");
     }
 
-    public void RemoveCert(ParseResult parseResult)
+    public int RemoveCert(ParseResult parseResult)
     {
         _logger.LogTrace("RemoveCert() called");
 
@@ -83,10 +83,17 @@ public void RemoveCert(ParseResult parseResult)
             var isForced = parseResult.GetValue(_forceOption);
             if (!isForced)
             {
+                if (Console.IsInputRedirected ||
+                    Environment.GetEnvironmentVariable("CI") is not null)
+                {
+                    _logger.LogError("Confirmation required but running in non-interactive mode. Use --force to skip confirmation.");
+                    return 1;
+                }
+
                 var isConfirmed = PromptConfirmation("Do you want to remove the root certificate", acceptByDefault: false);
                 if (!isConfirmed)
                 {
-                    return;
+                    return 0;
                 }
             }
 
@@ -111,10 +118,12 @@ public void RemoveCert(ParseResult parseResult)
             }
 
             _logger.LogInformation("DONE");
+            return 0;
         }
         catch (Exception ex)
         {
             _logger.LogError(ex, "Error removing certificate");
+            return 1;
         }
         finally
         {

DevProxy/Proxy/ProxyEngine.cs

@@ -231,8 +231,20 @@ private void FirstRunSetup()
 
         Console.WriteLine();
         Console.WriteLine("Dev Proxy uses a self-signed certificate to intercept and inspect HTTPS traffic.");
-        Console.Write("Update the certificate in your Keychain so that it's trusted by your browser? (Y/n): ");
-        var answer = Console.ReadLine()?.Trim();
+
+        string? answer;
+        if (Console.IsInputRedirected ||
+            Environment.GetEnvironmentVariable("CI") is not null)
+        {
+            // Non-interactive mode, default to trusting the certificate
+            _logger.LogInformation("Non-interactive mode detected. Defaulting to trusting the certificate.");
+            answer = "y";
+        }
+        else
+        {
+            Console.Write("Update the certificate in your Keychain so that it's trusted by your browser? (Y/n): ");
+            answer = Console.ReadLine()?.Trim();
+        }
 
         if (string.Equals(answer, "n", StringComparison.OrdinalIgnoreCase))
         {