ci: pin check-spelling to fix "Unsafe Permissions" (#929)

Use
check-spelling/check-spelling#103 (comment)
to workaround "Unsafe Permissions" failure
([example](https://github.com/a2aproject/a2a-python/actions/runs/23900666340)).

Author: ishymko

.github/workflows/spelling.yaml

@@ -27,7 +27,7 @@ jobs:
     steps:
       - name: check-spelling
         id: spelling
-        uses: check-spelling/check-spelling@main
+        uses: check-spelling/check-spelling@a35147f799f30f8739c33f92222c847214e82e67 # https://github.com/check-spelling/check-spelling/issues/103#issuecomment-4181666219
         with:
           suppress_push_for_open_pull_request: ${{ github.actor != 'dependabot[bot]' && 1 }}
           checkout: true