Fix lifecycle-spec in SchedulerPlacement (#1400)

Forgot that proto3 does not support optional booleans, despite it's
proto2-compatible syntax, so represent the requested lifecycle using a
(nullable) string instead.

modal/scheduler_placement.py

@@ -16,8 +16,12 @@ def __init__(
         spot: Optional[bool] = None,
     ):
         """mdmd:hidden"""
+        _lifecycle: Optional[str] = None
+        if spot is not None:
+            _lifecycle = "spot" if spot else "on-demand"
+
         self.proto = api_pb2.SchedulerPlacement(
             _region=region,
             _zone=zone,
-            _spot=spot,
+            _lifecycle=_lifecycle,
         )

modal_proto/api.proto

@@ -848,7 +848,7 @@ message SchedulerPlacement {
   //   GPU types.
   optional string _region = 1;
   optional string _zone = 2;
-  optional bool _spot = 3;
+  optional string _lifecycle = 3; // "on-demand" or "spot", else ignored
 }
 
 message FunctionHandleMetadata {

test/scheduler_placement_test.py

@@ -25,5 +25,5 @@ def test_scheduler_placement(servicer, client):
         assert fn._experimental_scheduler_placement == api_pb2.SchedulerPlacement(
             _region="us-east-1",
             _zone="us-east-1a",
-            _spot=False,
+            _lifecycle="on-demand",
         )