Apply some clang-tidy-19 suggestions

[Neverlord]

I meant to look into https://github.com/zeek/broker/issues/365 again to see if newer clang-tidy versions maybe fixed the issue. Turns out our clang-tidy CI check doesn't do anything anymore since we've moved our sources from src to libbroker. 🤦‍♂️

These are some changes clang-tidy-19 has suggested. Unfortunately, it did suggest changes that break the code, so we can't just update CI and start running this check with 19 again. Suppressing every suggestion that broke the code would add a lot of noise to the code. So before bringing the check back in a future PR, I'll see if I can maybe restructure the code slightly and otherwise disable noisy checks.