fix wal save crash and handle missing directory

[krushnarout]

omi/app/lib/utils/wal_file_manager.dart

Line 74 in 4df5a23

await _walFile!.writeAsString(jsonString);

here crash happened because the wal file written many times at the same time, fixed it by allowing only one file save at a time

PathNotFoundException: Cannot open file, path = '/Users/AVK/Documents/wals.json' (OS Error: No such file or directory, errno = 2)

also fixed this by creating the directory before writing

[gemini-code-assist]

Code Review

This pull request effectively addresses a crash from concurrent file writes by serializing save operations and preventing re-entrant flushing. It also handles a PathNotFoundException by ensuring the target directory exists before writing. My review focuses on improving the new error handling logic. I've pointed out two places where exceptions are being swallowed, which could lead to silent failures and subsequent issues. Specifically, I've suggested re-throwing an exception after logging it in _flush to ensure callers are aware of failures, and to fail early in saveWals if directory creation is unsuccessful.