fix(ci): lint test files on PRs without breaking push-to-main

The pr-review-poster was flagging `gtest/gtest.h file not found` on any
PR that added or modified a test file, because clang-tidy-diff-18.py
ran against files that weren't in the compilation database. PR #27004
and PR #26233 both hit this. The root cause is that test TUs only
enter compile_commands.json when BUILD_TESTING is ON, which the
historical clang-tidy build does not enable.

This PR fixes both halves of the problem:

1. Add a second make target `px4_sitl_default-clang-test` that configures
   a separate build dir with -DCMAKE_TESTING=ON. Test TUs land in its
   compile_commands.json with resolved gtest/fuzztest include paths.

2. Add an umbrella `clang-ci` target that depends on both
   `px4_sitl_default-clang` and `px4_sitl_default-clang-test` so the PR
   job prepares both build dirs with one make invocation.

3. On PR events the workflow uses `make clang-ci`, installs
   libclang-rt-18-dev (needed so fuzztest's FUZZTEST_FUZZING_MODE flags
   do not fail the abseil try_compile with a misleading "pthreads not
   found" error), and routes the clang-tidy-diff producer at the
   test-enabled build dir.

4. Push-to-main is left entirely alone: same single build dir, same
   `make px4_sitl_default-clang`, same `make clang-tidy`. Test files
   are not in that DB so run-clang-tidy.py keeps ignoring them exactly
   as before. This preserves green main while ~189 pre-existing
   clang-tidy issues in test files remain untouched; fixing those is
   out of scope for this change.

5. Replace the fragile `:!*/test/*` pathspec filter (which missed flat
   `*Test.cpp` files in module roots) with
   `Tools/ci/clang-tidy-diff-filter.py`, which reads the compilation
   database and drops any changed source file that is not a TU.
   Headers always pass through. Production code that happens to use
   test-like names (src/systemcmds/actuator_test, src/drivers/test_ppm,
   etc.) stays analyzed because those are real px4_add_module targets.

Verified in the ghcr.io/px4/px4-dev:v1.17.0-rc2 container and on the
real CI runner:
- cmake configure with CMAKE_TESTING=ON succeeds after installing
  libclang-rt-18-dev (Found Threads: TRUE)
- compile_commands.json grows from 1333 to 1521 TUs
- Modifying HysteresisTest.cpp with a new `const char *p = NULL`
  correctly flags hicpp-use-nullptr and
  clang-diagnostic-unused-variable on the new line, while pre-existing
  issues on other lines of the same file stay suppressed by
  clang-tidy-diff-18.py's line filter ("Suppressed ... 1 due to line
  filter")
- No gtest/gtest.h false positives
- Push-to-main path unchanged, still green

Signed-off-by: Ramon Roche <mrpollo@gmail.com>

Tools/ci/clang-tidy-diff-filter.py

@@ -0,0 +1,111 @@
+#!/usr/bin/env python3
+"""
+Filter a git diff for consumption by clang-tidy-diff.
+
+Produces a unified diff containing only files that clang-tidy can
+actually analyze against the current compilation database:
+
+  - C/C++ source files (.c, .cpp, .cc, .cxx, .m, .mm) must be present
+    in compile_commands.json. Files absent from the database are test
+    files, excluded code, or platform-specific sources that were not
+    compiled. Feeding them to clang-tidy-diff produces spurious
+    "header not found" errors (gtest/gtest.h in particular).
+
+  - Header files (.h, .hpp, .hxx) always pass through. clang-tidy
+    analyzes header changes via the TUs that include them; there is
+    no separate TU for a header to match against the database.
+
+  - All other files (CMakeLists.txt, .yml, .md, etc.) are dropped.
+
+Output is a unified diff suitable for piping into clang-tidy-diff.py.
+If nothing remains, the output file is empty.
+
+Used by .github/workflows/clang-tidy.yml as a pre-filter for the
+`pr-review` artifact producer. Python stdlib only.
+"""
+
+import argparse
+import json
+import os
+import subprocess
+import sys
+
+
+SOURCE_EXTS = {'.c', '.cpp', '.cc', '.cxx', '.m', '.mm'}
+HEADER_EXTS = {'.h', '.hpp', '.hxx'}
+
+
+def load_db_files(build_dir):
+    """Return the set of source paths (repo-relative) in compile_commands.json."""
+    path = os.path.join(build_dir, 'compile_commands.json')
+    with open(path) as f:
+        db = json.load(f)
+    root = os.path.abspath('.')
+    prefix = root + os.sep
+    paths = set()
+    for entry in db:
+        p = entry.get('file', '')
+        if p.startswith(prefix):
+            paths.add(p[len(prefix):])
+        else:
+            # Relative or external path; record as-is
+            paths.add(p)
+    return paths
+
+
+def changed_files(base_ref):
+    out = subprocess.check_output(
+        ['git', 'diff', '--name-only', '{}...HEAD'.format(base_ref)],
+        text=True,
+    )
+    return [line.strip() for line in out.splitlines() if line.strip()]
+
+
+def keep_file(path, db_files):
+    """Decide whether to keep this path in the filtered diff."""
+    ext = os.path.splitext(path)[1].lower()
+    if ext in HEADER_EXTS:
+        return True
+    if ext in SOURCE_EXTS:
+        return path in db_files
+    return False
+
+
+def filtered_diff(base_ref, keep_paths):
+    if not keep_paths:
+        return ''
+    cmd = ['git', 'diff', '-U0', '{}...HEAD'.format(base_ref), '--'] + sorted(keep_paths)
+    return subprocess.check_output(cmd, text=True)
+
+
+def main():
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument('--build-dir', required=True,
+                        help='CMake build dir containing compile_commands.json')
+    parser.add_argument('--base-ref', required=True,
+                        help='Git ref to diff against (e.g. origin/main)')
+    parser.add_argument('--out', required=True,
+                        help='Output path for the filtered unified diff')
+    args = parser.parse_args()
+
+    db_files = load_db_files(args.build_dir)
+    changed = changed_files(args.base_ref)
+
+    keep = [p for p in changed if keep_file(p, db_files)]
+    dropped = [p for p in changed if p not in keep]
+
+    print('clang-tidy-diff-filter: kept {} of {} changed files'.format(
+        len(keep), len(changed)))
+    if dropped:
+        print('  dropped (not in compile_commands.json or not source/header):')
+        for p in dropped:
+            print('    {}'.format(p))
+
+    diff = filtered_diff(args.base_ref, keep)
+    with open(args.out, 'w') as f:
+        f.write(diff)
+    return 0
+
+
+if __name__ == '__main__':
+    sys.exit(main())