Fix RunSecretProvider path issue

[jeanlaurent]

Fix path traversal vulnerability in RunSecretsProvider

Issue

The RunSecretsProvider.Get() method didn't validate secret names before constructing file paths, allowing path traversal attacks. Malicious secret names like ../../../etc/passwd could read arbitrary files accessible to the application.

Fix

• Added path validation using the existing path.ValidatePathInDirectory() function before reading files
• Blocks ../ sequences and absolute paths outside /run/secrets
• Returns empty string for invalid paths (consistent with existing error handling)
• Follows the same security pattern already used in env_files.go