Plugin tool needs to check for '@' in change log

The plugin tool crashes if a lone '@' is found in a file it rewrites. Having '@NotNull' in a commit message seem reasonable but causes the tool to crash.

Start by reproducing the plugin tool crash with a rewritten file or commit message containing a lone '@', such as '@NotNull', then trace the changelog rewrite path. Done means the tool no longer crashes when this character appears and the behavior is covered by an appropriate regression check.

Written by the indexing model from the issue text.