Enable compiler errors for implicit switch case fallthroughs.

Also unify warning options across Clang and GCC and fix minor issues that was surfaced by this.

PiperOrigin-RevId: 508619655
Change-Id: I59b777bf2dfea4422485670e2427c7d3f5cf6405

python/mujoco/errors.h

@@ -110,7 +110,8 @@ static thread_local std::jmp_buf mju_error_jmp_buf;
 static thread_local std::array<char, 1024> mju_error_msg{0};
 
 static inline void MjErrorHandler(const char* msg) {
-  std::strncpy(mju_error_msg.data(), msg, mju_error_msg.size());
+  std::strncpy(mju_error_msg.data(), msg, mju_error_msg.size() - 1);
+  mju_error_msg.data()[mju_error_msg.size() - 1] = '\0';
   std::longjmp(mju_error_jmp_buf, 1);
 }