[AppSignal E2E Testing] Validate E2E Tests Are Accounted For (#1126)

*Issue #, if available:*

*Description of changes:*
Add new validation workflow:
- This validation is to ensure that all ApplicationSignals e2e test
workflows relevant to this repo are actually being used in this repo.
- See:
https://github.com/aws-observability/aws-application-signals-test-framework/blob/main/.github/workflows/validate-e2e-tests-are-accounted-for.yml

*Testing:*
<img width="3138" height="1712" alt="image"
src="https://github.com/user-attachments/assets/1dfab8a9-362f-4d10-b206-e9ed2aa8ac54"
/>


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

Author: jj22ee

.github/workflows/application-signals-e2e-test.yml

@@ -245,3 +245,19 @@ jobs:
     with:
       aws-region: us-east-1
       caller-workflow-name: 'main-build'
+
+  # This validation is to ensure that all test workflows relevant to this repo are actually
+  # being used in this repo, which is referring to all the other jobs in this file.
+  #
+  # If this starts failing, then it most likely means that new e2e test workflow was
+  # added to `aws-observability/aws-application-signals-test-framework`, but was not
+  # added to this file. It could also mean that a test in this file has been removed.
+  #
+  # If a particular test file is intended to not be tested in this repo and should not
+  # be failing this particular validation, then choose one of the following options:
+  # - Add the test file to the exclusions input (CSV format) to the workflow
+  #   (see: https://github.com/aws-observability/aws-application-signals-test-framework/blob/main/.github/workflows/validate-e2e-tests-are-accounted-for.yml#L1)
+  # - Update the `validate-e2e-tests-are-accounted-for` job to change which "workflow files are expected to be used by this repo"
+  #   (see: https://github.com/aws-observability/aws-application-signals-test-framework/blob/main/.github/workflows/validate-e2e-tests-are-accounted-for.yml)
+  validate-all-tests-are-accounted-for:
+    uses: aws-observability/aws-application-signals-test-framework/.github/workflows/validate-e2e-tests-are-accounted-for.yml@main