Fix PowerShell -Command argv binding so positional args + stdin work

PowerShell's -Command treats trailing argv entries as part of the
command-line text rather than as $args, so a hook with an inline
command and an arg template raised a parser error. Wrap inline commands
in a scriptblock with @args splat, and pipe $input into the block so
{{body.*}} arg templates AND stdin JSON both reach the script. Verified
end-to-end against ping, bearer (good/bad/missing), HMAC (good/bad/missing),
IP allowlist deny, async 202, and stdin+template combined.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/WebhookServer.Core/Execution/ProcessExecutor.cs

@@ -160,7 +160,11 @@ public sealed class ProcessExecutor : IExecutor
         else
         {
             psi.ArgumentList.Add("-Command");
-            psi.ArgumentList.Add(endpoint.InlineCommand ?? "");
+            // Pipe stdin into a scriptblock so trailing argv entries bind via @args
+            // and the script can still consume the request body via $input.
+            // Without the wrapper, PowerShell concatenates all trailing args into the
+            // -Command string and fails to parse them.
+            psi.ArgumentList.Add("$input | & { " + (endpoint.InlineCommand ?? "") + " } @args");
         }
     }