refactor: type RunnerFactory.create_model/agent returns as Optional[Runner]

The factory's downstream consumers (ManagedModel, ManagedAgent) now
take Runner; aligning the factory's return types lets us drop the
type: ignore comments at the ManagedModel/ManagedAgent call sites.
Provider package PRs will update their concrete implementations to
match.

Judge still takes ModelRunner, so its call site picks up the
type: ignore[arg-type] in its place — that's resolved later in the
cleanup PR when Judge migrates to Runner.

Author: jsonbailey

packages/sdk/server-ai/src/ldai/client.py

@@ -339,9 +339,8 @@ def _create_judge_instance(
             if not provider:
                 return None
 
-            return Judge(judge_config, provider, sample_rate=sample_rate)
-        except Exception as e:
-            log.warning('Failed to initialize judge %r: %s', key, e)
+            return Judge(judge_config, provider, sample_rate=sample_rate)  # type: ignore[arg-type]
+        except Exception as error:
             return None
 
     def _build_evaluator(