refactor: single job to decide tests execution

[dvarasani-crest]

Description

This PR refactors the reusable-workflow to combine the test-inventory and setup-workflow as single job to decide which tests to be executed.

Checklist

• README.md has been updated or is not required
• push trigger tests
• manual release test
• automated releases test
• pull request trigger tests
• schedule trigger tests
• workflow errors/warnings reviewed and addressed

Testing done

docs-only: https://github.com/splunk/splunk-add-on-for-servicenow/actions/runs/17032952625
PR to branch other than main: https://github.com/splunk/splunk-add-on-for-servicenow/actions/runs/17118320373/job/48553559002
spl2: https://github.com/splunk/splunk-add-on-for-cisco-asa/actions/runs/17125634732
PR to main: https://github.com/splunk/splunk-add-on-for-servicenow/actions/runs/17032189425
PR to main with use_labels: https://github.com/splunk/splunk-add-on-for-servicenow/actions/runs/17034365647

[srv-rr-github-token]

🎉 This PR is included in version 5.1.0 🎉

The release is available on GitHub release

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