feat: ADDON-87374 Split FOSSA license and vulnerability checks in reusable release workflow

[nayanak-896]

Summary

This PR updates the reusable add-on build-test-release workflow to split FOSSA findings into separate license and vulnerability checks, improve release gating, and document the new workflow behavior.
JIRA Link - https://splunk.atlassian.net/browse/ADDON-87374

Changes

• Adds separate FOSSA CI jobs:
  • fossa-license-test
  • fossa-vulnerability-test
• Exposes separate pipeline outputs for:
  • active license issues
  • active vulnerability issues
  • release-blocking critical/high/medium vulnerability issues
• Publishes separate job summaries for license and vulnerability findings.
• Uploads separate raw FOSSA artifacts:
  • fossa-license-issues
  • fossa-vulnerability-issues
• Updates pre-publish release gating so release paths can block independently on:
  • unresolved critical/high/medium vulnerability issues
• Keeps merge-to-develop and other non-release workflows usable by reporting split FOSSA results without enforcing release gating there.
• Preserves the existing combined fossa-test behavior while split results are available.
• Updates README documentation for the new split FOSSA jobs and release gating behavior.

Checklist

• README.md has been updated
• push trigger tests
• manual release test
• automated releases test
• pull request trigger tests
• schedule trigger tests
• workflow errors/warnings reviewed and addressed

Tested TA GitHub Actions run:

1. Successful FOSSA Tests for Apache web server TA - https://github.com/splunk/splunk-add-on-for-apache-web-server/actions/runs/27413564545
2. Crowdstrike TA FOSSA tests failure - https://github.com/splunk/splunk-add-on-for-crowdstrike-fdr/actions/runs/27403537555
3. Salesforce TA FOSSA tests failure - https://github.com/splunk/splunk-add-on-for-salesforce/actions/runs/27403791531