chore(ci): request CLI integration test by label (#35171)

### Reason for this change

Enhance the CLI integration test workflow to provide more flexibility in triggering tests and better control over when integration tests run.

### Description of changes

- **Add explicit trigger types**: Specify explicit  event types (opened, synchronize, reopened, labeled, unlabeled) instead of using the default catch-all
- **Enable manual triggering**: Allow integration tests to be triggered manually via the `pr/request-cli-integ-tests` label, even when no CLI-related files have changed
- **Improve workflow logic**: Update the condition to check for both file changes OR the presence of the manual trigger label

These changes provide maintainers with better control over when integration tests run, especially useful for testing scenarios where CLI changes might not be automatically detected or when manual testing is needed.

### Describe any new or updated permissions being added

No new permissions are being added. This change only modifies workflow trigger conditions and logic.

### Description of how you validated changes

- Reviewed the workflow syntax and logic
- Verified that the label-based triggering condition is properly formatted
- Confirmed that existing functionality (automatic triggering on CLI file changes) remains intact

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: mrgrain

.github/workflows/request-cli-integ-test.yml

@@ -2,7 +2,13 @@
 
 name: request-cli-integ-test
 on:
-  pull_request_target: {}
+  pull_request_target:
+    types:
+      - opened
+      - synchronize
+      - reopened
+      - labeled
+      - unlabeled
 
 jobs:
   cli-changes:
@@ -28,7 +34,7 @@ jobs:
   submit-to-test-pipeline:
     environment: test-pipeline
     needs: cli-changes
-    if: needs.cli-changes.outputs.any-changed-files == 'true'
+    if: needs.cli-changes.outputs.any-changed-files == 'true' || contains(github.event.pull_request.labels.*.name, 'pr/request-cli-integ-tests')
     runs-on: ubuntu-latest
     steps:
       - name: Checkout