Add --describe flag to run_server for introspection support

Add a --describe CLI flag to run_server() that passes enable_describe=True
to RpcServer, allowing non-Python clients to use __describe__ introspection
over pipe and HTTP transports. Simplify serve_conformance_http.py to use
run_server instead of manual waitress setup.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: rustyconover

tests/serve_conformance_http.py

@@ -5,38 +5,16 @@
 
 Can be run directly: ``python -m tests.serve_conformance_http``
 
-Starts a waitress HTTP server exposing the conformance RPC service, prints
-``PORT:<n>`` to stdout so the parent process can discover the port.
+Starts an HTTP server exposing the conformance RPC service via ``run_server --http``.
 """
 
-import socket
-import sys
-
-import waitress
-
 from vgi_rpc.conformance import ConformanceService, ConformanceServiceImpl
-from vgi_rpc.http import make_wsgi_app
-from vgi_rpc.rpc import RpcServer
-
-
-def _find_free_port() -> int:
-    """Find a free TCP port on localhost."""
-    with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
-        s.bind(("127.0.0.1", 0))
-        return int(s.getsockname()[1])
+from vgi_rpc.rpc import run_server
 
 
 def main() -> None:
-    """Start a waitress HTTP server for the conformance service."""
-    port = int(sys.argv[1]) if len(sys.argv) > 1 else _find_free_port()
-
-    server = RpcServer(ConformanceService, ConformanceServiceImpl())
-    app = make_wsgi_app(server)
-
-    # Signal the port to the parent process
-    print(f"PORT:{port}", flush=True)
-
-    waitress.serve(app, host="127.0.0.1", port=port, _quiet=True)
+    """Start an HTTP server for the conformance service."""
+    run_server(ConformanceService, ConformanceServiceImpl())
 
 
 if __name__ == "__main__":

vgi_rpc/rpc/__init__.py

@@ -284,6 +284,7 @@ def run_server(protocol_or_server: type | RpcServer, implementation: object | No
       ``vgi-rpc[http]``).
     - ``--host HOST`` — HTTP bind address (default ``127.0.0.1``).
     - ``--port PORT`` — HTTP port (default ``0``, auto-select).
+    - ``--describe`` — Enable the ``__describe__`` introspection method.
 
     Without ``--http`` the server runs over stdin/stdout pipes (the
     default, suitable for ``SubprocessTransport``).
@@ -299,23 +300,26 @@ def run_server(protocol_or_server: type | RpcServer, implementation: object | No
         TypeError: On invalid argument combinations.
 
     """
+    parser = argparse.ArgumentParser(description="vgi-rpc server")
+    parser.add_argument("--http", action="store_true", default=False, help="Serve over HTTP instead of stdin/stdout")
+    parser.add_argument("--host", default="127.0.0.1", help="HTTP bind address (default: 127.0.0.1)")
+    parser.add_argument("--port", type=int, default=0, help="HTTP port (default: auto-select)")
+    parser.add_argument(
+        "--describe", action="store_true", default=False, help="Enable __describe__ introspection method"
+    )
+    args = parser.parse_args()
+
     if isinstance(protocol_or_server, RpcServer):
         if implementation is not None:
             raise TypeError("implementation must be None when passing an RpcServer")
         server = protocol_or_server
     elif isinstance(protocol_or_server, type):
         if implementation is None:
             raise TypeError("implementation is required when passing a Protocol class")
-        server = RpcServer(protocol_or_server, implementation)
+        server = RpcServer(protocol_or_server, implementation, enable_describe=args.describe)
     else:
         raise TypeError(f"Expected a Protocol class or RpcServer, got {type(protocol_or_server).__name__}")
 
-    parser = argparse.ArgumentParser(description="vgi-rpc server")
-    parser.add_argument("--http", action="store_true", default=False, help="Serve over HTTP instead of stdin/stdout")
-    parser.add_argument("--host", default="127.0.0.1", help="HTTP bind address (default: 127.0.0.1)")
-    parser.add_argument("--port", type=int, default=0, help="HTTP port (default: auto-select)")
-    args = parser.parse_args()
-
     if args.http:
         try:
             from vgi_rpc.http import serve_http