[bug/client codegen] local_discriminator should be stable (#104)

What changed and Why
===

When there are multiple discriminator choices, we should just choose the
first one.

Test plan
=========

Manual verification via repl

replit_river/codegen/client.py

@@ -193,7 +193,7 @@ def flatten_union(tpe: RiverType) -> list[RiverType]:
                             """.strip()
                             )
                             if local_discriminators:
-                                local_discriminator = local_discriminators.pop()
+                                local_discriminator = sorted(local_discriminators).pop()
                             else:
                                 local_discriminator = "FIXME: Ambiguous discriminators"
                             typeddict_encoder.append(