Pin style-check and pre-commit to clang-format 23.1.0

CI relied on ubuntu-latest's default apt clang-format (18.1.3), while the
pre-commit hook was pinned to a different 18.x build. Neither tracked a
specific, deliberately-chosen version, and the two could drift from each
other and from whatever a contributor has installed locally.

Both now pin the same clang-format 23.1.0 (latest stable), installed via
pipx in CI. Also scope the pre-commit hook's file matcher to the same set
of files test/Makefile's style_check target checks, since the previous
\.(cpp|cc|h)$ pattern reached into vendored code (test/gtest,
benchmark/crow) that must stay untouched.

Reformat httplib.h's brace-init spacing to match 23.1.0's output.

.github/workflows/test.yaml

@@ -37,6 +37,10 @@ jobs:
     steps:
       - name: checkout
         uses: actions/checkout@v4
+      - name: install clang-format
+        run: |
+          pipx install clang-format==23.1.0
+          echo "$HOME/.local/bin" >> "$GITHUB_PATH"
       - name: run style check
         run: |
           clang-format --version