fix(ci): fetch full history in changelog workflow so origin/master is available (#24898)

The shallow clone (depth=1) done by actions/checkout meant origin/master
did not exist, causing the check_changelog_fragments.sh script to fail
with "ambiguous argument 'origin/master'". Setting fetch-depth: 0 ensures
the full history and all remote refs are available.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: pront

.github/workflows/changelog.yaml

@@ -47,6 +47,8 @@ jobs:
       # Checkout PR branch (includes script and changelog.d/)
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         if: env.SHOULD_RUN == 'true'
+        with:
+          fetch-depth: 0
 
       - name: Run changelog fragment checker
         if: env.SHOULD_RUN == 'true'