CVE-2026-50267: Steeltoe: TLS private keys written to /tmp with default permissions, never deleted

When MySQL or PostgreSQL service bindings from VCAP_SERVICES include TLS client credentials, the Connectors library writes those credentials to temporary files in Path.GetTempPath() using File.CreateText. On Linux, File.CreateText creates files with mode 0644 (world-readable) under the process umask, and the files are never deleted. The same key material is protected at mode 0400 in /proc/<pid>/environ.