clang-tidy: Adapt Regex

Multiple issues with the existing header regex:

Problem 1: YAML round-trip corruption. The Python wrapper (clang_tidy.py) loads
the .clang-tidy file with yaml.safe_load, modifies the Checks field, then
writes it back with yaml.safe_dump. The dump produces:

HeaderFilterRegex: ^(?!external/.*).*

Note: no quotes. LLVM's YAML parser sees !external as a YAML tag indicator,
corrupting the regex. This silently disables header filtering — no headers get
checked for WarningsAsErrors violations.

Problem 2: Path mismatch. Even if the regex survived YAML, it anchors at
^external/. But Bazel sandbox paths look like
bazel-out/k8-fastbuild/bin/external/protobuf+/... — they don't start with
external/, so the negative lookahead never triggers.

The new config:

 - HeaderFilterRegex: '.*': check diagnostics from all headers -
 - ExcludeHeaderFilterRegex: clang-tidy 18+ feature that excludes
   matching headers:
    - (^|/)external/ — matches external/ anywhere in the path (handles
      Bazel's bazel-out/.../external/... paths)
    - \.json\.hh$ — excludes Seastar-generated swagger headers
      (auto-generated code we can't fix)

Neither value contains !, so they survive YAML round-tripping safely.

Author: StephanDollberg

.clang-tidy

@@ -1,7 +1,8 @@
 ---
 Checks:          'redpanda-*,clang-diagnostic-*,clang-analyzer-*,cert-*,cppcoreguidelines-*,hicpp-*,modernize-*,performance-*,misc-*,bugprone-*,-modernize-use-trailing-return-type,-modernize-use-nodiscard,-hicpp-named-parameter,-misc-include-cleaner,-clang-analyzer-optin.core.EnumCastOutOfRange,-cppcoreguidelines-avoid-reference-coroutine-parameters'
 WarningsAsErrors: 'redpanda-*,bugprone-use-after-move,bugprone-assert-side-effect,bugprone-assignment-in-if-condition,bugprone-dangling-handle,bugprone-sizeof-container,bugprone-stringview-nullptr,bugprone-unused-return-value,bugprone-suspicious-string-compare,misc-unused-using-decls,misc-unused-alias-decls,modernize-redundant-void-arg,performance-implicit-conversion-in-loop,performance-trivially-destructible,performance-no-automatic-move,performance-move-const-arg,bugprone-unused-local-non-trivial-variable'
-HeaderFilterRegex: '^(?!external/.*).*'
+HeaderFilterRegex: '.*'
+ExcludeHeaderFilterRegex: '(^|/)external/|\.json\.hh$'
 FormatStyle:    file
 CheckOptions:
   - key:             cert-dcl16-c.NewSuffixes