fix: Switch Codecov to OIDC auth, add unit-tests flag, scope push to main

JIRA: RHELWF-14239

Assisted-by: Claude Code (claude-opus-4-6) <noreply@anthropic.com>

Author: hluk

.github/workflows/gating.yaml

@@ -4,13 +4,18 @@ name: Gating
 "on":
   pull_request:
   push:
+    branches:
+      - main
   workflow_dispatch:
     inputs: {}
 
 jobs:
   tests:
     name: Unit tests
     runs-on: ubuntu-latest
+    permissions:
+      id-token: write
+      contents: read
 
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
@@ -29,7 +34,8 @@ jobs:
         with:
           fail_ci_if_error: true
           verbose: true
-          token: ${{ secrets.CODECOV_TOKEN }}
+          use_oidc: true
+          flags: unit-tests
 
   linters:
     name: Linters