tidy: clang-tidy is required

Otherwise this check will "pass", like:
```bash
-- Detecting CXX compile features - done
-- Found LLVM 21.1.8
-- Found clang-tidy: CLANG_TIDY_EXE-NOTFOUND
-- Configuring done (0.5s)
```

contrib/devtools/bitcoin-tidy/CMakeLists.txt

@@ -21,7 +21,7 @@ set(CMAKE_DISABLE_FIND_PACKAGE_ZLIB ON)
 set(CMAKE_DISABLE_FIND_PACKAGE_zstd ON)
 
 find_package(LLVM REQUIRED CONFIG)
-find_program(CLANG_TIDY_EXE NAMES "clang-tidy-${LLVM_VERSION_MAJOR}" "clang-tidy" HINTS ${LLVM_TOOLS_BINARY_DIR})
+find_program(CLANG_TIDY_EXE REQUIRED NAMES "clang-tidy-${LLVM_VERSION_MAJOR}" "clang-tidy" HINTS ${LLVM_TOOLS_BINARY_DIR})
 message(STATUS "Found LLVM ${LLVM_PACKAGE_VERSION}")
 message(STATUS "Found clang-tidy: ${CLANG_TIDY_EXE}")