fix(cli): --exclusively leads to 0 stacks being selected

The new concurrency logic introduced in #20345 was checking for all
dependencies to a stack to have been deployed previously for the stack
to be unblocked, but if not all stacks are selected, they will never
become unblocked.

Because there was no check on being "complete" at the end, this silently
leads to no stacks being deployed and the CLI no-opping.

Fix by only waiting for selected stacks among the dependencies, and
adding a safeguard at the end to make sure we deployed all stacks
successfully (or failed halfway through).

packages/aws-cdk/lib/deploy.ts

@@ -16,7 +16,7 @@ export const deployStacks = async (stacks: cxapi.CloudFormationStackArtifact[],
     stack.dependencies
       .map(({ id }) => id)
       .filter((id) => !id.endsWith('.assets'))
-      .every((id) => deploymentStates[id] === 'completed');
+      .every((id) => !deploymentStates[id] || deploymentStates[id] === 'completed'); // Dependency not selected or already finished
 
   const hasAnyStackFailed = (states: Record<string, DeploymentState>) => Object.values(states).includes('failed');
 
@@ -57,4 +57,10 @@ export const deployStacks = async (stacks: cxapi.CloudFormationStackArtifact[],
   if (deploymentErrors.length) {
     throw Error(`Stack Deployments Failed: ${deploymentErrors}`);
   }
+
+  // We shouldn't be able to get here, but check it anyway
+  const neverUnblocked = Object.entries(deploymentStates).filter(([_, s]) => s === 'pending').map(([n, _]) => n);
+  if (neverUnblocked.length > 0) {
+    throw new Error(`The following stacks never became unblocked: ${neverUnblocked.join(', ')}`);
+  }
 };