fix: pin check-spelling action to v0.0.26 (#1722)

# Description

- [ ] Follow the [`CONTRIBUTING`
Guide](https://github.com/a2aproject/A2A/blob/main/CONTRIBUTING.md).
- [ ] Make your Pull Request title in the
<https://www.conventionalcommits.org/> specification.
- [ ] Ensure the tests and linter pass (Run `nox -s format` from the
repository root to format)
- [ ] Appropriate docs were updated (if necessary)

Pins the `check-spelling/check-spelling` action to `v0.0.26` to resolve
the unsafe workflow configuration error (exit code 5) that was blocking
all PRs from passing the Check Spelling CI gate.

Fixes #1710 🦕

Signed-off-by: Sam Betts <1769706+Tehsmash@users.noreply.github.com>

Author: Tehsmash

.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@v0.0.26
         with:
           suppress_push_for_open_pull_request: ${{ github.actor != 'dependabot[bot]' && 1 }}
           checkout: true