refactor: single job to decide tests execution (#433)

### 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

- [x] `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

Author: dvarasani-crest