[pyright] [core] _cli/api (#11697)

### Summary & Motivation

Just massages some logic in `_cli/api` to fix a type error where some
variables were accessed (`cancellation_thread`) without them having a
guaranteed binding.

### How I Tested These Changes

BK

python_modules/dagster/dagster/_cli/api.py

@@ -95,6 +95,8 @@ def _execute_run_command_body(
         cancellation_thread, cancellation_thread_shutdown_event = start_run_cancellation_thread(
             instance, pipeline_run_id
         )
+    else:
+        cancellation_thread, cancellation_thread_shutdown_event = None, None
 
     pipeline_run: DagsterRun = check.not_none(
         instance.get_run_by_id(pipeline_run_id),
@@ -135,6 +137,8 @@ def _execute_run_command_body(
         run_worker_failed = True
     finally:
         if instance.should_start_background_run_thread:
+            cancellation_thread_shutdown_event = check.not_none(cancellation_thread_shutdown_event)
+            cancellation_thread = check.not_none(cancellation_thread)
             cancellation_thread_shutdown_event.set()
             if cancellation_thread.is_alive():
                 cancellation_thread.join(timeout=15)
@@ -198,6 +202,8 @@ def _resume_run_command_body(
         cancellation_thread, cancellation_thread_shutdown_event = start_run_cancellation_thread(
             instance, pipeline_run_id
         )
+    else:
+        cancellation_thread, cancellation_thread_shutdown_event = None, None
     pipeline_run = check.not_none(
         instance.get_run_by_id(pipeline_run_id),  # type: ignore
         "Pipeline run with id '{}' not found for run execution.".format(pipeline_run_id),
@@ -238,6 +244,8 @@ def _resume_run_command_body(
         run_worker_failed = True
     finally:
         if instance.should_start_background_run_thread:
+            cancellation_thread_shutdown_event = check.not_none(cancellation_thread_shutdown_event)
+            cancellation_thread = check.not_none(cancellation_thread)
             cancellation_thread_shutdown_event.set()
             if cancellation_thread.is_alive():
                 cancellation_thread.join(timeout=15)