chore(repo): add .serena to .gitignore (#5464)

### Description

When Serena AI is used in the repository, it creates a directory
within the project containing project memories (where it
stores the results of indexing the project).

Adding this to the `.gitignore` so as to avoid accidentally committing
it in the future.

#### Issues
No issue associated with this change

Author: ericapisani

.gitignore

@@ -29,6 +29,7 @@ pip-wheel-metadata
 .mypy_cache
 .vscode/
 .claude/
+.serena
 .tool-versions
 
 # for running AWS Lambda tests using AWS SAM