fix(platform): small robustness gaps in openDirectoryInFileManager and the Mongo TLS temp key #1768

• `openDirectoryInFileManager` (lib/core/platform/open_directory.dart): `dir.create(recursive: true)` is outside the `try`, while the contract says it returns `false` on failure. An unwritable or invalid path throws instead.
• `resolveMongoTlsCertificateKeyFile` (lib/core/security/ssl_certificate_support.dart): the merged PEM is written with the default mode and only then `chmod 600`. The directory is `700`, so the window is narrow, but the file should be created with restrictive permissions from the start. If `writeAsString` throws, the temp directory is left behind.
• `cleanupStaleMongoTlsTempFiles` deletes every `querya_mongo_tls_*` directory in the system temp at start-up, including those of another running Querya instance; its next Mongo reconnect then loses the certificate file.

Found in the system-functions review (see #1764).