Add attachments support to G4.Plugins.Common/Actions/SendSmtpMail.cs

[gravity-api]

Add attachments support to G4.Plugins.Common/Actions/SendSmtpMail.cs.

Acceptance criteria

• Plugin accepts attachments via parameters
• Support raw base64 file input
• Attachments are added to outgoing MailMessage.Attachments and disposed correctly
• Invalid attachment input is handled gracefully
• Unit or integration tests cover attachments