Add error handling for OTLP/gRPC listener binding in trace command (#748)

cli/test/test_trace_cmd.py

@@ -0,0 +1,44 @@
+import pytest
+import rich_click as click
+
+from planoai import trace_cmd
+
+
+class _FakeGrpcServer:
+    def add_insecure_port(self, _address: str) -> int:
+        raise RuntimeError("bind failed")
+
+    def start(self) -> None:
+        return None
+
+
+def test_create_trace_server_raises_bind_error(monkeypatch):
+    monkeypatch.setattr(
+        trace_cmd.grpc, "server", lambda *_args, **_kwargs: _FakeGrpcServer()
+    )
+    monkeypatch.setattr(
+        trace_cmd.trace_service_pb2_grpc,
+        "add_TraceServiceServicer_to_server",
+        lambda *_args, **_kwargs: None,
+    )
+
+    with pytest.raises(trace_cmd.TraceListenerBindError) as excinfo:
+        trace_cmd._create_trace_server("0.0.0.0", 4317)
+
+    assert "already in use" in str(excinfo.value)
+    assert "planoai trace listen --port" in str(excinfo.value)
+
+
+def test_start_trace_listener_converts_bind_error_to_click_exception(monkeypatch):
+    monkeypatch.setattr(
+        trace_cmd,
+        "_create_trace_server",
+        lambda *_args, **_kwargs: (_ for _ in ()).throw(
+            trace_cmd.TraceListenerBindError("port in use")
+        ),
+    )
+
+    with pytest.raises(click.ClickException) as excinfo:
+        trace_cmd._start_trace_listener("0.0.0.0", 4317)
+
+    assert "port in use" in str(excinfo.value)