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.

.pre-commit-config.yaml

@@ -1,7 +1,7 @@
 repos:
   - repo: https://github.com/pre-commit/mirrors-clang-format
-    rev: v18.1.8  # 最新バージョンを使用
+    rev: v23.1.0  # CIのstyle-checkと同じバージョンに揃える
     hooks:
       - id: clang-format
-        files: \.(cpp|cc|h)$
+        files: ^(httplib\.h|(example|fuzzing|test)/[^/]+\.(cpp|cc|h))$
         args: [-i]  # インプレースで修正