Fix CI failures: replace gh API calls blocked by IP allow list (#1363)

## Summary
After migrating to GitHub-hosted runners (#1350), the `releaseFreeze`
and `checkNextChangelog` workflows fail with:
```
GraphQL: the databricks organization has an IP allow list enabled,
and your IP address is not permitted to access this resource.
```

GitHub-hosted runner IPs are not in the Databricks org allow list, so
`gh pr view` GraphQL calls are blocked.

## Fix
Replace `gh` CLI API calls with data already available locally:
- **PR body**: Use `github.event.pull_request.body` from the event
payload instead of `gh pr view --json body`
- **Changed files**: Use `git diff --name-only` between base and head
SHAs instead of `gh pr view --json files`

No API calls needed — all data comes from the event payload or local
git.

## Test plan
- [x] The PR itself will exercise both workflows when opened

NO_CHANGELOG=true

---------

Signed-off-by: Gopal Lal <gopal.lal@databricks.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: gopalldb

.github/workflows/release-thin.yml

@@ -95,14 +95,21 @@ jobs:
 
       - name: Verify main release exists
         if: github.event_name == 'workflow_run'
-        run: |
-          echo "Verifying main release exists for ${{ steps.get_tag.outputs.tag }}"
-          gh release view "${{ steps.get_tag.outputs.tag }}" || {
-            echo "Error: Main release not found for tag ${{ steps.get_tag.outputs.tag }}"
-            exit 1
-          }
-        env:
-          GH_TOKEN: ${{ github.token }}
+        uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea # v7
+        with:
+          script: |
+            const tag = '${{ steps.get_tag.outputs.tag }}';
+            console.log(`Verifying main release exists for ${tag}`);
+            try {
+              await github.rest.repos.getReleaseByTag({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                tag: tag
+              });
+              console.log(`Release found for ${tag}`);
+            } catch (e) {
+              core.setFailed(`Main release not found for tag ${tag}: ${e.message}`);
+            }
 
       - name: Upload Thin JAR to GitHub Release
         uses: softprops/action-gh-release@de2c0eb89ae2a093876385947365aca7b0e5f844 # v1