fix(security): restrict permissions and cleanup temp tls pem for mongodb (#915)

[ZhuchkaTriplesix]

Summary

• Restrict POSIX file permissions (0600 file / 0700 dir) when creating merged temporary client TLS PEM keys in resolveMongoTlsCertificateKeyFile.
• Add cleanupMongoTlsTempFile and track temporary client PEM in MongoConnection, ensuring reliable cleanup upon disconnect or connection errors.
• Add cleanupStaleMongoTlsTempFiles hook executed on app startup in lib/main.dart to purge any leftover temporary TLS files from previous sessions.
• Add unit tests covering restrictive permissions, targeted cleanup, and connection error cleanup.

Linked issue

Closes #915

Checklist

• flutter analyze passes locally
• Scope matches the linked issue only (no drive-by refactors)
• PR targets dev